Field sequential image display device and image display method

ABSTRACT

With a field sequential method, image display having sufficiently high color reproduction is performed, and an occurrence of color breakup is more reliably suppressed. In a field sequential liquid crystal display apparatus in which a plurality of subframes including a red subframe, a green subframe, a blue subframe, and a white (common color) subframe constitutes each frame, an image data conversion unit (30) converts input image data (D1) corresponding to red, green, and blue into driving image data (D2) corresponding to the plurality of subframes, based on an adjustment coefficient (Ks) and a distribution ratio (WRs) for the white subframe, for each pixel. Two distribution ratios are selectively used as the distribution ratio (WRs). A function of obtaining a second distribution ratio (WRsv2) of the two distribution ratios is set such that the distribution ratio (WRsv2) increases as the adjusted brightness (brightness after amplification and compression processing) (V) for an input image becomes smaller.

TECHNICAL FIELD

The present invention relates to an image display device, andparticularly, to a field sequential image display device and a fieldsequential liquid image display method.

BACKGROUND ART

In the related art, a field sequential image display device thatdisplays a plurality of subframes in one frame period is known. Forexample, a typical field sequential image display device includes abacklight including a red light source, a green light source, and a bluelight source, and displays red, green, and blue subframes in one frameperiod. When a red subframe is displayed, a display panel is drivenbased on red image data, and the red light source emits light. A greensubframe and a blue subframe are displayed in the similar manner. Threesubframes displayed in a time division manner are combined on theretinae of an observer by an afterimage phenomenon, and thus theobserver recognizes these subframes as one color image.

In the field sequential image display device, when the eyeline of theobserver moves in a display screen, a situation in which the observerlooks as if the colors of the subframes are separated from each othermay occur (this phenomenon is referred to as “color breakup”). In orderto suppress the occurrence of color breakup, an image display devicethat displays a white subframe in addition to the red, green, and bluesubframes is known. An image display device that performs amplificationprocessing of multiplying input image data by one or more coefficientswhen driving image data including red image data, green image data, blueimage data, and white image data is obtained based on the input imagedata including red image data, green image data, and blue image data isknown.

Relating to an image display device disclosed in this application, PTLs1 and 2 disclose a method of obtaining driving image data including redimage data, green image data, blue image data, and white image databased on input image data including red image data, green image data,and blue image data, in an image display device which includes subpixelsof red, green, blue and white colors and is not the field sequentialtype.

CITATION LIST Patent Literature

-   PTL 1: Japanese Unexamined Patent Application Publication No.    2001-147666-   PTL 2: Japanese Unexamined Patent Application Publication No.    2008-139809-   PTL 3: Japanese Unexamined Patent Application Publication No.    2010-33009-   PTL 4: Japanese Unexamined Patent Application Publication No.    2002-229531

SUMMARY OF INVENTION Technical Problem

In the above-described field sequential image display device, a whitesubframe is provided as a common color subframe for preventing theoccurrence of color breakup, and driving image data is generated byimage-data conversion processing including amplification processing ofmultiplying input image data by one or more coefficients, a differencein hue, saturation, and luminance may occur between a color (referred toas “an extended input color” below) indicated by image data subjected tothe amplification processing and a color (referred to as “an actualdisplay color” below) which is actually displayed in a display devicesuch as a liquid crystal panel. In this case, image display havingsufficiently high color reproduction is not performed.

In the above-described field sequential image display device, dependingon an image to be displayed, a distribution ratio for the white subframeis not sufficient in conversion from input image data to driving imagedata. Thus, reliable suppression of the occurrence of color breakup maynot be possible.

Thus, it is desired to provide a field sequential image display deviceand a field sequential image display method in which it is possible toperform image display having sufficiently high color reproduction and tomore reliably suppress the occurrence of color breakup.

Solution to Problem

According to a first aspect of the present invention, there is provideda field sequential image display device in which a plurality of subframeperiods including a plurality of primary-color subframe periodsrespectively corresponding to a plurality of primary colors and at leastone common-color subframe period is included in each frame period. Thefield sequential image display device includes an image data conversionunit that receives input image data corresponding to the plurality ofprimary colors and generates driving image data corresponding to theplurality of subframe periods from the input image data by obtaining apixel data value of each of the plurality of subframe periods for eachpixel of an input image represented by the input image data, based onthe input image data, and a display unit that displays an image based onthe driving image data.

The image data conversion unit generates the driving image data byconversion processing in which, for each pixel in the input image, anadjustment coefficient to be multiplied by a value of the pixel and acommon-color distribution ratio are determined, and a pixel data valuein each of the plurality of subframe periods is obtained from the valueof the pixel based on the adjustment coefficient and the common-colordistribution ratio, the common-color distribution ratio being defined asa ratio of a display light quantity of a common color component, whichis to be emitted in the common-color subframe period to a display lightquantity of the common color component, which is to be emitted in oneframe period for displaying the pixel.

In the conversion processing, for each pixel in the input image, thecommon-color distribution ratio is determined in accordance with thesaturation and an adjusted brightness of the pixel such that thecommon-color distribution ratio increases as a hue and the saturation inan HSV space are maintained, and the adjusted brightness decreases, andthe pixel is allowed to be displayed in the display unit, the adjustedbrightness being a brightness after the value of the pixel is multipliedby the adjustment coefficient.

According to a second aspect of the present invention, in the firstaspect of the present invention, for each pixel in the input image, theimage data conversion unit determines a tentative distribution ratiocorresponding to the ratio in accordance with the saturation of thepixel such that the pixel data value in the common-color subframe periodis greater than a minimum value of pixel data values in the plurality ofprimary-color subframe periods and smaller than a maximum value thereof,and determines the common-color distribution ratio based on thetentative distribution ratio in accordance with the brightness of thepixel.

According to a third aspect of the present invention, in the firstaspect of the present invention, the image data conversion unit includesa parameter storage unit that stores a parameter used in the conversionprocessing, and the parameter storage unit stores a parameter inaccordance with response characteristics in image display in the displayunit.

According to a fourth aspect of the present invention, in the thirdaspect of the present invention, the image data conversion unit furtherstores a parameter for designating a range of a maximum value inaccordance with a minimum value of pixel data values of each pixel inthe input image in the plurality of subframe periods.

According to a fifth aspect of the present invention, in the third orfourth aspect of the present invention, the display unit includes alight source unit that emits light having a corresponding color in eachsubframe period, a light modulation unit that cause the light from thelight source unit to be transmitted therethrough or be reflectedthereby, a light-source-unit driving circuit that drives the lightsource unit to irradiate the light modulation unit with the light havingthe corresponding color in each subframe period, and alight-modulation-unit driving circuit that controls transmittance orreflectance in the light modulation unit such that an image of thecorresponding color in each subframe period is displayed.

The parameter storage unit further stores a light emission controlparameter, and the light-source-unit driving circuit controls lightemission luminance of the common color in the light source unit based onthe light emission control parameter.

According to a sixth aspect of the present invention, in the firstaspect of the present invention, the image data conversion unit obtainsthe common-color distribution ratio in accordance with a function havinga value which smoothly changes depending on the saturation.

According to a seventh aspect of the present invention, in the firstaspect of the present invention, the image data conversion unitgenerates the driving image data by conversion processing in which, foreach pixel in the input image, the adjustment coefficient is determinedbased on pixel data values in the plurality of subframe periods inaccordance with the saturation of the pixel in a range in which thepixel is allowed to be displayed in the display unit, and the pixel datavalue in each of the plurality of subframe periods is obtained from thevalue of the pixel based on the adjustment coefficient and thecommon-color distribution ratio.

According to an eighth aspect of the present invention, in the seventhaspect of the present invention, the image data conversion unit obtainsthe common-color distribution ratio and the adjustment coefficient inaccordance with functions having a value which smoothly changesdepending on the saturation.

According to a ninth aspect of the present invention, in the seventhaspect of the present invention, the image data conversion unitdetermines the adjustment coefficient and the common-color distributionratio such that a maximum value is linearly limited with respect to aminimum value among pixel data values in the plurality of subframeperiods, for each pixel in the input image.

According to a tenth aspect of the present invention, in the seventhaspect of the present invention, the image data conversion unit assumesa function of the saturation, which indicates a tentative coefficientfor obtaining the adjustment coefficient and a function of thesaturation, which indicates a correction coefficient to be multiplied bythe tentative coefficient, and obtains a multiplication result of thetentative coefficient and the correction coefficient based on thesaturation of the pixel for each pixel in the input image, as theadjustment coefficient.

The correction coefficient is set such that a rate of the adjustmentcoefficient changing with respect to the saturation when the saturationis equal to or smaller than a predetermined value is equal to or smallerthan a predetermined value.

According to an eleventh aspect of the present invention, in the firstaspect of the present invention, the image data conversion unit includesa parameter storage unit that stores a parameter used in the conversionprocessing, and the display unit includes a temperature sensor, theparameter storage unit stores a plurality of values for the parameter,in accordance with a temperature, and the image data conversion unitselects the value in accordance with the temperature measured by thetemperature sensor among the plurality of values stored in the parameterstorage unit and uses the selected value in the conversion processing.

According to a twelfth aspect of the present invention, in the firstaspect of the present invention, the image data conversion unit includesa frame memory that stores the input image data, and generates thedriving image data corresponding to a pixel, based on the input imagedata which has been stored in the frame memory and corresponds to aplurality of pixels, for each pixel in the input image.

According to a thirteenth aspect of the present invention, in the firstaspect of the present invention, the image data conversion unit performsthe conversion processing on normalized luminance data.

According to a fourteenth aspect of the present invention, in the firstaspect of the present invention, the image data conversion unit obtainsthe driving image data by performing response compensation processing onimage data obtained after the conversion processing.

According to a fifteenth aspect of the present invention, in the firstaspect of the present invention, the plurality of primary colorsincludes blue, green, and red, and the common color is white.

According to a sixteenth aspect of the present invention, there isprovided a field sequential image display method in which a plurality ofsubframe periods including a plurality of primary-color subframe periodsrespectively corresponding to a plurality of primary colors and at leastone common-color subframe period is included in each frame period.

The method includes an image-data conversion step of receiving inputimage data corresponding to the plurality of primary colors andgenerating driving image data corresponding to the plurality of subframeperiods from the input image data by obtaining a pixel data value ofeach of the plurality of subframe periods for each pixel of an inputimage represented by the input image data, based on the input imagedata, and a display step of displaying an image based on the drivingimage data.

The image-data conversion step includes a coefficient-and-distributionratio determination step of determining an adjustment coefficient to bemultiplied by a value of the pixel and determining a common-colordistribution ratio defined as a ratio of a display light quantity of acommon color component, which is to be emitted in the common-colorsubframe period to a display light quantity of the common colorcomponent, which is to be emitted in one frame period for displaying thepixel, for each pixel in the input image, and a driving image-dataoperation step of generating the driving image data by conversionprocessing of obtaining the pixel data value in each of the plurality ofsubframe periods from the value of the pixel based on the adjustmentcoefficient and the common-color distribution ratio, for each pixel inthe input image.

In the coefficient-and-distribution ratio determination step, for eachpixel in the input image, the common-color distribution ratio isdetermined in accordance with the saturation and an adjusted brightnessof the pixel such that the common-color distribution ratio increases asa hue and the saturation in an HSV space are maintained, and theadjusted brightness decreases, and the pixel is allowed to be displayedin the display unit, the adjusted brightness being a brightness afterthe value of the pixel is multiplied by the adjustment coefficient.

Other aspects of the present invention are clear from descriptionsregarding the first to sixteenth aspects of the present invention andembodiments described later, and thus descriptions thereof will beomitted.

Advantageous Effects of Invention

According to the first aspect of the present invention, the common-colordistribution ratio is determined in accordance with the saturation andthe adjusted brightness of a pixel such that the common-colordistribution ratio increases as the hue and the saturation in the HSVspace are maintained, and the adjusted brightness becomes smaller, foreach pixel in an input image indicated by input image data. Sincedriving image data is generated based on such a common-colordistribution ratio, it is possible to suppress the occurrence of colorbreakup while image display having high color reproduction is performed.

According to the second aspect of the present invention, for each pixelin the input image, the tentative distribution ratio is obtained suchthat the pixel data value in the common-color subframe period is greaterthan the minimum value of the pixel data values in the plurality ofprimary-color subframe periods and smaller than the maximum value. Thecommon-color distribution ratio increasing as the brightness becomessmaller is obtained based on the tentative distribution ratio. Thus, itis possible to suppress the occurrence of color breakup while imagedisplay having high color reproduction is performed.

According to the third aspect of the present invention, it is possibleto improve color reproduction by setting the suitable parameter inaccordance with the response characteristics of the display unit.

According to the fourth aspect of the present invention, the maximumvalue of driving image data in one frame period is limited in accordancewith the minimum value of the driving image data in one frame period, byusing the parameter stored in the parameter storage unit. Thus, it ispossible to improve color reproduction.

According to the fifth aspect of the present invention, it is possibleto reduce heat generated in the light source by controlling theluminance of the light source when a common color subframe is displayedin a field sequential image display device that includes the displayunit using the light modulation unit that cause light from the lightsource to be transmitted therethrough or be reflected thereby.

According to the sixth aspect of the present invention, the common-colordistribution ratio is obtained in accordance with the function whichsmoothly changes depending on the saturation. Thus, it is possible toprevent the occurrence of distortion of a gradation image when the imageis displayed. Thus, it is possible to perform image display having highcolor reproduction.

According to the seventh aspect of the present invention, the tentativedistribution ratio is obtained, for each pixel in the input image, suchthat the pixel data value in the common-color subframe period is greaterthan the minimum value of the pixel data values in the plurality ofprimary-color subframe periods and smaller than the maximum valuethereof. In addition, for each pixel in the input image, the adjustmentcoefficient to be multiplied by the value of the pixel is obtained basedon the pixel data values in the plurality of subframe periods, in arange in which the pixel is allowed to be displayed in the display unit,in accordance with the saturation of the pixel. The common-colordistribution ratio increasing as the brightness becomes smaller isobtained with the tentative distribution ratio and the adjustmentcoefficient which have obtained in the above-described manner. Thus, themore preferable common-color distribution ratio depending on theadjustment coefficient is obtained. Thus, it is possible to suppress theoccurrence of color breakup while image display having high colorreproduction is performed.

According to the eighth aspect of the present invention, thecommon-color distribution ratio and the adjustment coefficient areobtained in accordance with the functions which smoothly changedepending on the saturation. Thus, it is possible to prevent theoccurrence of distortion of a gradation image when the image isdisplayed. Thus, it is possible to perform image display having highcolor reproduction.

According to the ninth aspect of the present invention, the maximumvalue of the driving image data in one frame period is linearly limitedwith respect to the minimum value of the driving image data in the oneframe period. Thus, the range of the maximum value is determined inaccordance with the minimum value. Thus, it is possible to suppress achange of the image data after the conversion, in one frame period, andto improve color reproduction of the image display device.

According to the tenth aspect of the present invention, the amount ofthe adjustment coefficient changing with respect to the change of thesaturation S is small even though the saturation S is small, and theluminance is high (see an H2 portion in FIG. 16). Since the drivingimage data is generated based on such an adjustment coefficient, it ispossible to suppress an occurrence of gradation skipping occurring in adisplay image, and to suppress an occurrence of noise occurring at ahigh-luminance portion included in the display image.

According to the eleventh aspect of the present invention, theconversion processing is performed based on the parameter in accordancewith the temperature of the display unit. Thus, it is possible toimprove color reproduction even in a case where the responsecharacteristics of the display unit change in accordance with thetemperature.

According to the twelfth aspect of the present invention, the conversionprocessing is performed based on the input image data corresponding tothe plurality of pixels. Thus, it is possible to prevent the occurrenceof a situation in which the color of a pixel rapidly changes in aspatial direction or a time direction.

According to the thirteenth aspect of the present invention, theconversion processing is performed on normalized luminance data. Thus,it is possible to accurately perform the conversion processing.

According to the fourteenth aspect of the present invention, theresponse compensation processing is performed on image data after theconversion processing has been performed. Thus, it is possible todisplay a desired image even in a case where the response rate of thedisplay unit is slow.

According to the fifteenth aspect, in the image display device thatdisplays subframes of three primary colors and the white color based onthe input image data corresponding to the three primary colors, it ispossible to improve color reproduction.

Effects in other aspects of the present invention are clearly obtainedfrom the effects in the first to fifteenth aspects of the presentinvention and the following descriptions of embodiments. Thus,descriptions thereof will not be repeated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of an imagedisplay device according to a first embodiment.

FIG. 2 is a diagram illustrating a parameter in the image display deviceaccording to the first embodiment.

FIG. 3 is a flowchart illustrating image-data conversion processing inthe image display device according to the first embodiment.

FIG. 4 is a diagram illustrating a range of a saturation and adistribution ratio of a white subframe in the image display deviceaccording to the first embodiment.

FIG. 5 is a diagram illustrating a graph of a first distribution ratioWRs1 according to a first example of the first embodiment.

FIG. 6 is a diagram illustrating a graph of a first distribution ratioWRs1 according to a second example of the first embodiment.

FIG. 7 is a diagram illustrating a graph of a first distribution ratioWRs1 according to a first modification example of the first embodiment.

FIG. 8 is a diagram illustrating a graph of an adjustment coefficient Ksaccording to the first modification example of the first embodiment.

FIG. 9 is a diagram illustrating a graph of a first distribution ratioWRs1 according to a second modification example of the first embodiment.

FIG. 10 is a diagram illustrating a graph of an adjustment coefficientKs according to the second modification example of the first embodiment.

FIG. 11 is a diagram illustrating a method of determining a function ofobtaining a second distribution ratio WRsv2 of a white subframe in theimage display device according to the first embodiment.

FIGS. 12(A) and 12(B) are diagrams illustrating an operation of aconventional example as a comparative example for describing effects ofthe first embodiment.

FIGS. 13(A) and 13(B) are diagrams illustrating the effects of the firstembodiment.

FIG. 14 is a block diagram illustrating a configuration of an imagedisplay device according to a second embodiment.

FIG. 15 is a flowchart illustrating image-data conversion processing inthe image display device according to the second embodiment.

FIG. 16 is a diagram illustrating a graph for an adjustment coefficientKs in the image display device according to the second embodiment.

FIGS. 17(A) to 17(C) are diagrams illustrating graphs of a coefficientKsv in a case where low-luminance-portion noise handling processing isperformed in the second embodiment.

FIG. 18 is a diagram illustrating a range allowed to be taken by thecoefficient Ksv in a case where the low-luminance-portion noise handlingprocessing is performed in the second embodiment.

FIG. 19 is a diagram illustrating a range allowed to be taken by a valueNS in a case where the low-luminance-portion noise handling processingis performed in the second embodiment.

FIG. 20 is a diagram illustrating a graph of the value NS set in a casewhere the low-luminance-portion noise handling processing is performedin the second embodiment.

FIG. 21 is a diagram illustrating graphs of the coefficients Ksv and Ks,which are used for describing effects of the low-luminance-portion noisehandling processing in the second embodiment.

FIG. 22 is a diagram illustrating an example of image-data conversionprocessing in a case where the low-luminance-portion noise handlingprocessing is not performed in the second embodiment.

FIG. 23 is a diagram illustrating an example of the image-dataconversion processing in a case where the low-luminance-portion noisehandling processing is performed in the second embodiment.

FIG. 24 is a diagram illustrating a method of determining a function ofobtaining a second distribution ratio WRsv2 in the image display deviceaccording to the second embodiment.

FIG. 25 is a diagram illustrating a graph illustrating a firstdistribution ratio and a parameter WRZ for describing the method ofdetermining the function of obtaining the second distribution ratioWRsv2 in the image display device according to the second embodiment.

FIG. 26 is a block diagram illustrating a configuration of an imagedisplay device according to a third embodiment.

FIG. 27 is a block diagram illustrating a configuration of an imagedisplay device according to a fourth embodiment.

FIG. 28 is a block diagram illustrating a configuration of an imagedisplay device according to a modification example of the secondembodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, image display devices and image display methods accordingto embodiments will be described with reference to the drawings.Firstly, the following is noted. “Computation” provided in the followingdescriptions includes the meaning that “a computation result is storedin a table in advance, and the computation result is obtained based onthe table”, in addition to the meaning of “obtaining a computationresult with a computing machine”.

1. First Embodiment

<1.1 Overall Configuration>

FIG. 1 is a block diagram illustrating a configuration of an imagedisplay device according to a first embodiment. An image display device1 illustrated in FIG. 1 includes an image data conversion unit 10 and adisplay unit 20. The image data conversion unit 10 includes a parameterstorage unit 11, a statistical value-and-saturation computation unit 12,a distribution ratio-and-coefficient computation unit 13, and a drivingimage-data operation unit 33. The display unit 20 includes a timingcontrol circuit 21, a panel driving circuit 22, a backlight drivingcircuit 23, a liquid crystal panel 24 as a light modulation unit, and abacklight 25 as a light source unit.

The image display device 1 is a field sequential liquid crystal displayapparatus. The image display device 1 divides one frame period into aplurality of subframes periods and displays a different color subframein each of the subframe periods. Hereinafter, it is assumed that theimage display device 1 divides one frame period into four subframeperiods and respectively displays white, blue, green, and red subframesin first to fourth subframe periods. In the image display device 1, awhite subframe is a common color subframe. “The color” in each subframeindicates a light source color. It is assumed that the display unit 20in the image display device 1 can display “a white color” as a desiredcolor temperature in a case where “1” (maximum value) is assigned to anyof a red color, a green color, and a blue color in light-source drivingdata used for driving the backlight 25 (also similar in otherembodiments described later).

Input image data D1 including red image data, green image data, and blueimage data is input to the image display device 1. The image dataconversion unit 10 obtains driving image data D2 corresponding to white,blue, green, and red subframes, based on the input image data D1. Theprocessing is referred to as “image-data conversion processing” below.Pieces of the driving image data D2 corresponding to white, blue, green,and red subframes are referred to as “white image data, blue image data,green image data, and red image data which are included in the drivingimage data D2”, respectively. The display unit 20 displays the white,blue, green, and red subframes in one frame period, based on the drivingimage data D2.

The timing control circuit 21 outputs a timing control signal TC to thepanel driving circuit 22 and the backlight driving circuit 23. The paneldriving circuit 22 drives the liquid crystal panel 24 based on thetiming control signal TC and the driving image data D2. The backlightdriving circuit 23 drives the backlight 25 based on the timing controlsignal TC and a parameter WBR (which will be described later) from theparameter storage unit 11. The liquid crystal panel 24 includes aplurality of pixels 26 arranged in two dimensions. The backlight 25includes a red light source 27 r, a green light source 27 g, and a bluelight source 27 b (the light sources 27 r, 27 g, and 27 b are alsocollectively referred to as “a light source 27” below). The backlight 25may include a white light source. For example, a light emitting diode(LED) is used as the light source 27.

In the first subframe period, the panel driving circuit 22 drives theliquid crystal panel 24 based on white image data included in thedriving image data D2, and the backlight driving circuit 23 causes thered light source 27 r, the green light source 27 g, and the blue lightsource 27 b to emit light. Thus, a white subframe is displayed. In acase where the backlight 25 includes a white light source, the backlightdriving circuit 23 may cause the white light source to emit light in thefirst subframe period.

In the second subframe period, the panel driving circuit 22 drives theliquid crystal panel 24 based on blue image data included in the drivingimage data D2, and the backlight driving circuit 23 causes the bluelight source 27 b to emit light. Thus, a blue subframe is displayed. Inthe third subframe period, the panel driving circuit 22 drives theliquid crystal panel 24 based on green image data included in thedriving image data D2, and the backlight driving circuit 23 causes thegreen light source 27 g to emit light. Thus, a green subframe isdisplayed. In the fourth subframe period, the panel driving circuit 22drives the liquid crystal panel 24 based on red image data included inthe driving image data D2, and the backlight driving circuit 23 causesthe red light source 27 r to emit light. Thus, a red subframe isdisplayed.

<1.2 Details of Image Data Conversion Unit>

Details of the image data conversion unit 10 will be described below.Red image data, green image data, and blue image data (red, green, andblue are original color components) which are included in the inputimage data D1 are luminance data normalized to have a value of 0 to 1.When pieces of image data of three colors are equal to each other, thepixel 26 becomes achromatic. Red image data, green image data, and blueimage data which are included in the driving image data D2 are alsoluminance data normalized to have a value of 0 to 1. For example, amicrocomputer including a central processing unit (CPU) and a memory maybe used as the image data conversion unit 10. The image data conversionunit 10 may be realized in software by the microcomputer executing apredetermined program corresponding to FIG. 3 described later. Instead,the entirety or some components of the image data conversion unit 10 maybe realized with dedicated hardware (typically, application specificintegrated circuit designed to be dedicated).

In image-data conversion processing, amplification and compressionprocessing and color-component conversion processing are performed (seeExpressions (5a) to (5d) described later). The amplification andcompression processing is performed with an adjustment coefficient Kswhich is a coefficient to be multiplied by the values (referred to as“BGR pixel data values of an input image” below) of the blue color, thegreen color, and the red color in each pixel of an image (input image)representing input image data D1. In the color-component conversionprocessing, the BGR pixel data value of the input image subjected to theamplification and compression processing is converted into pixel datavalues (referred to as “driving WBGR pixel data values” below) of awhite subframe, a blue subframe, a green subframe, and a red subframe.In the image-data conversion processing, white image data (having avalue to be distributed to a common color subframe) included in thedriving image data D2 is determined in a range of 0 to 1. In theimage-data conversion processing, for each pixel in an input image, aratio (referred to as “a distribution ratio WRs of a white subframe” or“a common-color distribution ratio WRs” below, or simply referred to as“a distribution ratio WRs” below) of the display light quantity of awhite-color component to be emitted in a white subframe period to thedisplay light quantity of the white-color component to be emitted in oneframe period for displaying the pixel is firstly determined. Then, thedriving WGBR pixel data values of white image data are obtained based onthe distribution ratio WRs. For example, in a case where the adjustmentcoefficient Ks is determined to 1 and the distribution ratio WRs isdetermined to be 0.6 when red image data included in input image data D1is 0.5, and green image data and blue image data are 1, white image dataincluded in driving image data D2 is 0.3. In the embodiment as describedlater, the luminance of the light source 27 when the white subframe isdisplayed is controlled to be WBR times the luminance of the lightsource 27 when other subframes are displayed, in accordance with theparameter WBR. Therefore, a relation between the pixel data value in awhite subframe period and display luminance by this pixel data valuedepends on the parameter WBR. Considering this point, the common-colordistribution ratio WRs is more accurately defined as a ratio of a valueobtained by a product of the white image data and the parameter WBR inthe driving image data D2, to the minimum value of the BGR pixel datavalues of the input image subjected to the amplification and compressionprocessing.

The parameter storage unit 11 stores parameters WRX, WRZ, VCBU, RA, RB,and WBR used in image-data conversion processing. The statisticalvalue-and-saturation computation unit 12 obtains the maximum value Dmax,the minimum value Dmin, and the saturation S based on input image dataD1, for each pixel. The distribution ratio-and-coefficient computationunit 13 obtains the distribution ratio WRs and an adjustment coefficient(also simply referred to as “a coefficient” below) Ks based on themaximum value Dmax, the saturation S, and the parameters WRX, WRZ, VCBU,RA, RB, and WBR (details will be described later). The drivingimage-data operation unit 33 obtains driving image data D2 based on theinput image data D1, the minimum value Dmin, the distribution ratio WRs,the coefficient Ks, and the parameter WBR.

The parameters stored in the parameter storage unit 11 will be describedbelow. The parameter WRX is a parameter depending on responsecharacteristics of a pixel 26 provided in the display unit 20. Theparameter WRX is included in a calculation expression of obtaining thedistribution ratio WRs. The parameter WBR designates the luminance ofthe light source 27 which is used when a white subframe is displayed andis provided in the backlight 25, and takes a value in a range of0≤WBR≤1.

The minimum value of driving image data D2 in one frame period is set asDDmin, and the maximum value thereof is set as DDmax. The distributionratio-and-coefficient computation unit 13 obtain the coefficient Ks inaccordance with the parameters RA and RB stored in the parameter storageunit 11, so as to satisfy the following expression (1).

DDmax≤RA·DDmin+RB  (1)

For example, in a case of RB=1−RA, the range satisfying the expression(1) corresponds to a shaded area illustrated in FIG. 2. As describedabove, the parameters RA and RB designate the range of the maximum valueDDmax in accordance with the minimum value DDmin. As represented byExpression (1), the range of the maximum value of driving image data inone frame period is determined in accordance with the minimum value ofthe driving image data in the one frame period. Thus, it is possible tosuppress the change of image data after conversion in one frame periodand to improve color reproduction.

As described above, the parameter WBR designates the luminance of thelight source 27 which is used when a white subframe is displayed and isprovided in the backlight 25, and takes a value in a range of 0≤WBR≤1.The display unit 20 controls the luminance of the light source 27 inaccordance with the parameter WBR, when displaying a white subframe.More specifically, the backlight driving circuit 23 in the display unit20 controls the luminance of the light source 27 when a white subframeis displayed, to be WBR times the luminance of the light source 27 whenother subframes are displayed, in accordance with the parameter WBR.

FIG. 3 is a flowchart illustrating image-data conversion processing. Theprocessing illustrated in FIG. 3 is performed on data of each pixel,which is included in input image data D1. Processing on image data Ri,Gi, and Bi of three colors will be described below on the assumptionthat red image data, green image data, and blue image data (BGR pixeldata values of an input image) of a pixel, which are included in inputimage data D1 are respectively set as Ri, Gi, and Bi, and white imagedata (driving WBGR pixel data values), blue image data, green imagedata, and red image data of the pixel, which are included in drivingimage data D2 are respectively set as Wd, Bd, Gd, and Rd.

As illustrated in FIG. 3, the image data Ri, Gi, and Bi of three colorsare input to the image data conversion unit 10 (Step S101). Then, thestatistical value-and-saturation computation unit 12 obtains the maximumvalue Dmax and the minimum value Dmin of the image data Ri, Gi, and Biof the three colors (Step S102). Then, the statisticalvalue-and-saturation computation unit 12 obtains a saturation S by thefollowing expression (2), based on the maximum value Dmax and theminimum value Dmin (Step S103).

S=(Dmax−Dmin)/Dmax  (2)

Here, in the expression (2), S is set to 0 when Dmax is 0.

The distribution ratio-and-coefficient computation unit 13 obtains adistribution ratio of a white subframe by a calculation expression(which will be described later), based on the saturation S and theparameter WRX (Step S104). The distribution ratio obtained at this timeis referred to as “a first distribution ratio WRs1” below.

The distribution ratio-and-coefficient computation unit 13 obtains thecoefficient Ks by a calculation expression (which will be describedlater) based on the saturation S and the parameters WRX, RA, RB, and WBR(Step S105). When the distribution ratio-and-coefficient computationunit 13 obtains the first distribution ratio WRs1 in Step S104 andobtains the coefficient Ks in Step S105, the distributionratio-and-coefficient computation unit 13 obtains the maximum value (ora value smaller than the maximum value) allowed to be taken by thecoefficient Ks under a condition in which the first distribution ratioWRs1 is used, and the maximum value Dmax of the input image data D1 isset as Maximum Value 1 allowed to be taken by the input image data D1.

Then, the distribution ratio-and-coefficient computation unit 13 obtainsa second distribution ratio WRsv2 by an expression (which will bedescribed later) based on the saturation S, the maximum value Dmax, thefirst distribution ratio WRs1, the coefficient Ks, and the parametersRA, RB, WBR, and WRZ (Step S201).

Then, the distribution ratio-and-coefficient computation unit 13performs condition branching in accordance with the parameter VCBU (StepS202). The parameter VCBU indicates whether or not color-breakuphandling processing is performed, and takes a value of 0 or 1. The valueof 0 indicates that the color-breakup handling processing is notperformed. The value of 1 indicates that the color-breakup handlingprocessing is performed. The distribution ratio-and-coefficientcomputation unit 13 causes the process to Step S203 at time of VCBU=0and to proceed to Step S204 at time of VCBU=1. In the former case, thedistribution ratio-and-coefficient computation unit 13 outputs the firstdistribution ratio WRs1 obtained in Step S104, as a distribution ratioWRs of a white subframe (Step S203). In the latter case, thedistribution ratio-and-coefficient computation unit 13 outputs thesecond distribution ratio WRsv2 obtained in Step S201, as thedistribution ratio of the white subframe (common-color distributionratio) WRs (Step S204).

The driving image-data operation unit 33 obtains image data Wd, Bd, Gd,and Rd of four colors based on the image data Ri, Gi, and Bi of thethree colors, the minimum value Dmin, the distribution ratio WRs, thecoefficient Ks, and the parameter WBR by the following expressions (5a)to (5d) (Step S106).

Wd=WRs·Dmin·Ks·PP/WBR  (5a)

Bd=(Bi−WRs·min)Ks·PP  (5b)

Gd=(Gi−WRs·Dmin)Ks·PP  (5c)

Rd=(Ri−WRs·Dmin)Ks·PP  (5d)

Here, in the expressions (5a) to (5d), PP indicates a value (=P/Pmax)obtained by dividing the maximum value P for gradation constraint by themaximum value Pmax of the gradation. In the following descriptions, PP=1is assumed.

The driving image-data operation unit 33 obtains the image data Wd, Bd,Gd, and Rd of the four colors based on the first distribution ratioWRs1, at time of VCBU=0. The driving image-data operation unit 33obtains the image data Wd, Bd, Gd, and Rd of the four colors based onthe second distribution ratio WRsv2, at time of VCBU=1. In this manner,the image data conversion unit 10 does not perform color-breakuphandling processing at time of VCBU=0, and performs the color-breakuphandling processing at time of VCBU=1.

<1.3 Method of Determining Function of Obtaining First DistributionRatio WRs1>

The saturation S and the distribution ratio WRs take values of 0 to 1.The maximum value of blue image data Bd, green image data Gd, and redimage data Rd which are included in the driving image data D2 is set asDdmax, and the minimum value thereof is set as Ddmin. When PP is 1, Wd,Ddmax, and Ddmin are given by the following expressions (6a) to (6c),respectively.

Wd=WRs·Dmin·Ks/WBR  (6a)

Ddmax=(Dmax−WRs·Dmin)Ks  (6b)

Ddmin=(Dmin−WRs·Dmin)Ks  (6c)

The following expression (7a) is derived by solving the expression ofWd>Ddmax in consideration of Dmax=Dmin/(1−S). The following expression(7b) is derived by solving the expression of Wd<Ddmin.

WRs>WBRo/(1−S)  (7a)

WRs<WBRo  (7b)

Here, in the expressions (7a) and (7b), WBRo satisfies WBR/(1+WBR).

FIG. 4 is a diagram illustrating a range of the saturation S and thedistribution ratio WRs. The range of (S, WRs) illustrated in FIG. 4 isdivided into a first area in which Ddmin<Wd<Ddmax is satisfied, a secondarea in which Ddmax<Wd is satisfied, and a third area in which Wd<Ddminis satisfied.

As described above, in a case where color-breakup handling processing isnot performed (in a case of VCBU=0), the first distribution ratio WRs1is used as the distribution ratio WRs of the white subframe (Steps S202and S203 in FIG. 3). The distribution ratio-and-coefficient computationunit 13 has a function of obtaining the first distribution ratio WRs1based on the saturation S. The function changes depending on theparameters WRX, RA, RB, and WBR stored in the parameter storage unit 11.Details of the function will be described below. In a first example anda second example described below, the parameter WRX takes a value in arange of ½≤WRX≤1.

In the first example, the distribution ratio-and-coefficient computationunit 13 obtains the first distribution ratio WRs1 by the followingexpression (17).

WRs1=min(WBRo/(1−S),WRX)  (17)

In the expression (17), WBRo is ½.

FIG. 5 is a diagram illustrating a graph of the first distribution ratioWRs1 according to the first example. The graph illustrated in FIG. 5 isnormally in the first area. It is possible to set a difference betweenimage data Wd, Bd, Gd, and Rd of the four colors to be the minimum, byusing the first distribution ratio WRs1 as the distribution ratio WRs ofthe white subframe.

In a case where the response rate of the pixel 26 becomes slower as thedisplay gradation becomes lower, the parameter WRX is set to a valueclose to 1, and the white image data Wd is set to approach the maximumvalue Ddmax. In a case where the response rate of the pixel 26 becomesslower as the display gradation becomes higher, the parameter WRX is setto a value close to 0.5, and the white image data Wd is set to approachthe minimum value Ddmin. As described above, if the parameter WRX is setin accordance with the response characteristics of the pixel 26, it ispossible to improve color reproduction of the image display device 1 bydisplaying the gradation with the higher response rate.

In the second example, the distribution ratio-and-coefficientcomputation unit 13 obtains the first distribution ratio WRs1 by thefollowing expressions (18a) to (18c).

a) Time of WRX≥Ts and 1−S<WBRx

WRs1=WRX−WRX(1−S)²/(3·WBRx ²)  (18a)

b) Time of WRX≥Ts and 1−S≥WBRx

WRs1=WBRo/(1−S)  (18b)

c) Time of WRX<Ts

WRs1=(WBRo−WRX)(1−S)² +WRX  (18c)

In the expressions (18a) to (18c), WBRo is ½, Ts is ¾, and WBRx is3/(4WRX).

FIG. 6 is a diagram illustrating a graph of the first distribution ratioWRs1 according to the second example. A function of obtaining the firstdistribution ratio WRs1 at time of WRX<Ts(=¾) is a quadratic functionwhich takes a value of WBRo(=½) at time of S=0 and takes the maximumvalue WRX at time of S=1. A function of obtaining the first distributionratio WRs1 at time of WRX≥Ts is a fractional function WRs1=1/{2(1−S)} attime of 1−S≥WBRx and is a quadratic function which takes a value ofmaximum value WRX at time of 1−S<WBRx and S=1. The latter function isdetermined such that the graph of the former function is tangent to thegraph of the latter function at a point (WBRx, WBRo/(1−WBRx)). The graphillustrated in FIG. 6 is normally in the first area. It is possible toset a difference between image data Wd, Bd, Gd, and Rd of the fourcolors to be the minimum, by using such a first distribution ratio WRs1as the distribution ratio WRs of the white subframe.

In the second example, in a case where the response rate of the pixel 26becomes slower as the display gradation becomes lower, the parameter WRXis also set to a value close to 1. In a case where the response rate ofthe pixel 26 becomes slower as the display gradation becomes higher, theparameter WRX is also set to a value close to 0.5. Thus, similar to thefirst example, it is possible to improve color reproduction of the imagedisplay device 1.

In the first example, when WRX is not 0.5, the function of obtaining thefirst distribution ratio WRs1 does not smoothly change in the vicinityof S=1−WBRo/WRX. In the second example, the function smoothly changes ina range of 0<S<1. Thus, according to the second example, it is possibleto prevent distortion of an image when a gradation image is displayed.

Features and effects of a case where the first distribution ratio WRs1is used as the distribution ratio WRs of the white subframe in the firstexample and the second example will be described below. In the first andsecond examples, the distribution ratio-and-coefficient computation unit13 obtains the distribution ratio WRs so as to cause (S, WRs) to be inthe first area. The first area indicates a range satisfyingDdmin<Wd<Ddmax, that is, a range in which white image data Wd is in arange from the minimum value Ddmin of blue image data Bd, green imagedata Gd, and red image data Rd to the maximum value Ddmax thereof. Asdescribed above, since the distribution ratio WRs is obtained so as tocause the white image data Wd to be in the range from the minimum valueDdmin of blue image data Bd, green image data Gd, and red image data Rdto the maximum value Ddmax thereof, it is possible to suppress thechange of luminance of the pixel 26 in one frame period and to improvecolor reproduction of the image display device 1.

In the first and second examples, the distribution ratio-and-coefficientcomputation unit 13 obtains the distribution ratio WRs increasing as thesaturation S becomes greater. Thus, it is possible to suppress theoccurrence of color breakup by setting the ratio of the valuedistributed to the white subframe to increase as the saturation Sbecomes greater.

In the second example, the distribution ratio-and-coefficientcomputation unit 13 obtains the distribution ratio WRs in accordancewith the function which smoothly changes depending on the saturation S.Thus, it is possible to prevent distortion of an image when a gradationimage is displayed. In this specification, “the function that smoothlychanges” means, for example, a function of a differential coefficientcontinuously changing. However, it is not limited thereto. The functionmay be a smooth function without an inflection point. That is, in a casewhere, even though the differential coefficient of a function isdiscontinuous, a problem on display does not occur because the extent ofdiscontinuity is sufficient small, this function may be considered as“the function that smoothly changes”.

In the first example and the second example, at time of S=0, WRs is 0.5,and Wd=Bd=Gd=Rd is established. As described above, the distributionratio-and-coefficient computation unit 13 obtains the distribution ratioWRs such that the white image data Wd, the blue image data Bd, the greenimage data Gd, and the red image data Rd are equal to each other at timeof S=0. Thus, it is possible to prevent an occurrence of a situation inwhich the luminance of the pixel 26 in one frame period changes when thesaturation S is zero.

<1.4 Regarding Adjustment Coefficient Ks>

The adjustment coefficient Ks will be described below (Step S104 in FIG.3).

As illustrated in FIG. 4 and described above, the range of (S, WRs),which is indicated by the saturation S and the distribution ratio WRs isdivided into the first area in which Ddmin<Wd<Ddmax is satisfied, thesecond area in which Ddmax<Wd is satisfied, and the third area in whichWd<Ddmin is satisfied.

In a case where (S, WRs) is in the first area, DDmin is Ddmin, and DDmaxis Ddmax. Considering the expressions (6a) and (6b), if the expression(1) is solved by substituting Dmin=Dmax(1−S) into the expression (1),the following expression (20) is derived.

Ks≤RB/(Dmax×[1−{WRs(1−RA)+RA}(1−S)])  (20)

The coefficient Ks is determined as with the following expression (21)so as to establish the expression (20) even when the maximum value Dmaxis 1 (maximum value which may be taken by the input image data D1). Theexpression (21) represents the maximum value which may be taken by thecoefficient Ks under a condition of Dmax=1, in a case where (S, WRs) isin the first area.

Ks=RB/[1−{WRs(1−RA)+RA}(1−S)]  (21)

The maximum value Dmax indicates a brightness Vi of the input image dataD1. The brightness Vi=Dmax=max(Ri, Gi, Bi) may be referred to as “aninput brightness Vi” below in order to be distinguished from thebrightness V=Ks·Dmax after amplification and compression processingdescribed later.

In a case where the distribution ratio WRs is determined to cause (S,WRs) to be in the first area, the expression of Ddmin<Wd<Ddmax isestablished, and a difference between image data Wd, Bd, Gd, and Rd offour colors included in the driving image data D2 becomes the minimum(even in a case of the maximum, (Ddmax−Ddmin) is established). In thiscase, the maximum value which may be taken by the coefficient Ks under acondition in which the distribution ratio WRs is used and Dmax is 1 isgiven by the expression (21). As (S, WRs) becomes closer to a boundaryline between the first and second areas, the white image data Wdapproaches the maximum value Ddmax. As (S, WRs) becomes closer to aboundary line between the first and third areas, the white image data Wdapproaches the minimum value Ddmin.

In a case where (S, WRs) is in the second area, DDmin is Ddmin, andDDmax is Wd. Considering the above expressions, the expressions (6a) and(6c), and Dmin=Dmax(1−S), the following expression (22) by theexpression (1) is derived.

Ks≤WBR·RB/[Dmax(1−S){WRs(1+WBR·RA)−RA)−RA·WBR}]  (22)

The coefficient Ks is determined as with the following expression (23)so as to establish the expression (22) even when the maximum value Dmaxindicating the input brightness Vi is 1 (maximum value which may betaken by the input image data D1). The expression (23) represents themaximum value which may be taken by the coefficient Ks under a conditionof Dmax=1, in a case where (S, WRs) is in the second area.

Ks=WBR·RB/[{WRs(1+WBR·RA)−RA·WBR}(1−S)]  (23)

In a case where (S, WRs) is in the third area, DDmin is Wd, and DDmax isDdmax. Considering the expressions, the expressions (6a) and (6b), andDmin=Dmax(1−S), the following expression (24) by the expression (1) isderived.

Ks≤WBR·RB/[Dmax{WBR−(WBR+RA)WRs(1−S)}]  (24)

The coefficient Ks is determined as with the following expression (25)so as to establish the expression (24) even when the maximum value Dmaxindicating the input brightness Vi is 1 (maximum value which may betaken by the input image data D1). The expression (24) represents themaximum value which may be taken by the coefficient Ks under a conditionof Dmax=1, in a case where (S, WRs) is in the third area.

Ks=WBR·RB/{WBR−(WBR+RA)WRs(1−S)}  (25)

<1.5 First Distribution Ratio WRs1 and Adjustment Coefficient KsAccording to Modification Examples>

Another examples of the function of obtaining the distribution ratio WRsand the function of obtaining the coefficient Ks will be describedbelow, as modification examples. The parameters RA, RB, and WBR havevalues in ranges of 0≤RA≤1, 0≤RB≤1, and 0≤WBR≤1, respectively. In afirst modification example and a second modification example describedbelow, the parameter WRX takes a value in a range of WBRo≤WRX≤1.

In the first modification example, the distributionratio-and-coefficient computation unit 13 obtains the first distributionratio WRs1 by the expression (26) and obtains the coefficient Ks by theexpression (21).

WRs1=min(WBRo/(1−S),WRX)  (26)

In the expression (26), WBRo satisfies WBR/(1+WBR). FIG. 7 is a diagramillustrating a graph of the first distribution ratio WRs1 according tothe first modification example. FIG. 8 is a diagram illustrating a graphof the coefficient Ks according to the first modification example. Inthe graphs illustrated in FIGS. 7 and 8, RA=0.25, RB=0.75, and WBR=0.5are set. According to the first modification example, that is, if thecoefficient Ks in the expression (21) is used, and the firstdistribution ratio WRs1 in the expression (26) is set as thedistribution ratio WRs of the white subframe, effects similar to thosein the first example in the embodiment are obtained.

In the second modification example, the distributionratio-and-coefficient computation unit 13 obtains the first distributionratio WRs1 by the expressions (18a) to (18c) and obtains the coefficientKs by the expression (21). In the expressions (18a) to (18c), WBRo isWBR/(1+WBR), Ts is 3WBRo/2, and WBRx is 3WBR/{2WRX(1+WBR)}. FIG. 9 is adiagram illustrating a graph of the first distribution ratio WRs1according to the second modification example. FIG. 10 is a diagramillustrating a graph of the coefficient Ks according to the secondmodification example. In the graphs illustrated in FIGS. 9 and 10,RA=0.25, RB=0.75, and WBR=0.5 are set. According to the secondmodification example, that is, if the coefficient Ks in the expression(21) is used, and the first distribution ratio WRs1 in the expressions(18a) to (18c) is set as the distribution ratio WRs of the whitesubframe, effects similar to those in the second example in theembodiment are obtained.

<1.6 Method of Determining Function of Obtaining Second DistributionRatio WRsv2>

A calculation expression of obtaining the second distribution ratioWRsv2 will be described below (Step S201 in FIG. 3). In the followingdescriptions, calculation is performed in consideration of V=Dmax·Ks andDmin=Dmax(1−S). Here, Dmax indicates the input brightness Vi, and Vindicates a brightness after amplification and compression processing(also referred to as “an adjusted brightness” below). In a case where(S, WRs1) is in the first area illustrated in FIG. 4, DDmin is Ddmin,and DDmax is Ddmax. Thus, the minimum value WRsva of the seconddistribution ratio WRsv2 in this case is given by the followingexpression (27a) with Ddmax≤RA·Ddmin+RB. In a case where (S, WRs1) is inthe second area illustrated in FIG. 4, DDmin is Ddmin, and DDmax is Wd.Thus, the maximum value WRsvb of the second distribution ratio WRsv2 inthis case is given by the following expression (27b) withWd≤RA·Ddmin+RB. In a case where (S, WRs1) is in the third areaillustrated in FIG. 4, DDmin is Wd, and DDmax is Ddmax. Thus, theminimum value WRsvc of the second distribution ratio WRsv2 in this caseis given by the following expression (27c) with Ddmax≤RA·Wd+RB.

WRsva=RA/(RA−1)+(RB−V)/{(RA−1)V(1−S)}  (27a)

WRsvb=WBR·RA/(1+WBR·RA)+WBR·RB/{(1+WBR·RA)V(1−S)}  (27b)

WRsvc=WBR(V−RB)/{(WBR+RA)V(1−S)}  (27c)

When the second distribution ratio WRsv2 is obtained, in a case where(S, WRs1) is in the first area illustrated in FIG. 4, the seconddistribution ratio WRsv2 is set to be equal to or greater than the valueWRsva shown in the expression (27a). In a case where (S, WRs1) is in thesecond area illustrated in FIG. 4, the second distribution ratio WRsv2is set to be equal to or smaller than the value WRsvb shown in theexpression (27b). In a case where (S, WRs1) is in the third areaillustrated in FIG. 4, the second distribution ratio WRsv2 is set to beequal to or greater than the value WRsvc shown in the expression (27c).Considering that the second distribution ratio WRsv2 takes a value in arange of 0≤WRsv2≤1, the second distribution ratio WRsv2 is determined tosatisfy the following expression (28).

max(0,WRsva,WRsvc)≤WRsv2≤min(1,WRsvb)  (28)

FIG. 11 is a diagram illustrating a method of determining a function ofobtaining WRsv2. A curved line illustrated by a solid line in FIG. 11indicates the second distribution ratio WRsv2 at time of S=0.5. Here,RA=0.25, RB=0.75, WBR=0.5, and WRX=0.75 are set. The distributionratio-and-coefficient computation unit 13 obtains the first distributionratio WRs1 by the expressions (18a) to (18c) and obtains the coefficientKs by the expression (21).

A portion surrounded by a bold broken line in FIG. 11 indicates a rangeof the adjusted brightness (brightness after amplification andcompression processing) V and the second distribution ratio WRsv2satisfying the expression (28). The function of obtaining the seconddistribution ratio WRsv2 is determined such that the graph of thefunction is in the range surrounded by the bold broken line illustratedin FIG. 11.

For example, the function of obtaining the second distribution ratioWRsv2 when the coefficient Ks is smaller than a predetermined value Tsvis set to be a quadratic function which takes the maximum value WRZ attime of V=0 and takes a value WRs1 at time of V=Ks. The function ofobtaining the second distribution ratio WRsv2 when the coefficient Ks isequal to or greater than the predetermined value Tsv is set to be aquadratic function which takes the maximum value WRZ at time of V=0under a condition of V<Tsv. This function is set to be a fractionalfunction which can be expressed as WRsv2=A/V+B under a condition ofV≥Tsv and takes the value WRs1 at time of V=Ks (A indicatesWBR·RB/{(1+WBR/RA) (1−S)}). The following expressions (29) and (30a) to(30c) are derived by determining two functions cause the graphs of thetwo functions to be tangent to each other at time of V=Tsv (WRsv2 is setto WRs1 at time of WRZ≤WRs1).

Tsv=3·Ks·WBRb/[2{Ks(1−S)(WRZ−WRs1)+WBRb}]  (29)

a) Time of Ks≥Tsv and V≥Tsv

WRsv2=WRs1+WBRb(Ks−V)/{Ks(1−S)V}  (30a)

b) Time of Ks≥Tsv and V<Tsv

WRsv2=WRZ−WBRb·V2/{2(1−S)Tsv3}  (30b)

c) Time of Ks<Tsv

WRsv2=WRZ−(WRZ−WRs1)V2/Ks2  (30c)

In the expressions (29) and (30a) to (30c), WBRb=WBR·RB/(1+WBR·RA) andV=Dmax·Ks are established. The parameter WRZ takes a value in a range ofWRs≤WRZ≤1. V indicates the adjusted brightness (brightness afteramplification and compression processing).

As illustrated in FIG. 11, the second distribution ratio WRsv2 increasesas the adjusted brightness V becomes smaller. Therefore, at time ofVCBU=1, the distribution ratio-and-coefficient computation unit 13obtains the distribution ratio WRs increasing as the adjusted brightnessV becomes smaller. Thus, it is possible to increase an effect ofpreventing the occurrence of color breakup, when the adjusted brightnessV is small. At time of VCBU=1, the parameter WRZ is set to a valuecloser to the first distribution ratio WRs1 as the response rate of thepixel 26 becomes slower, and is set to a value closer to 1 as theresponse rate of the pixel 26 becomes faster. Thus, it is possible toobtain the preferable distribution ratio WRs in accordance with theresponse rate of the pixel 26 and to suppress the occurrence of colorbreakup.

<1.7 Effects of First Embodiment>

As described above, in the image display device 1 according to theembodiment, at time of VCBU=1, the distribution ratio-and-coefficientcomputation unit 13 obtains the tentative distribution ratio (firstdistribution ratio WRs1) and the coefficient Ks based on the saturationS and the parameters WRX, RA, RB, and WBR and obtains the distributionratio WRs(second distribution ratio WRsv2) increasing as the adjustedbrightness V becomes smaller, based on the adjusted brightness V(obtained by multiplying the maximum value Dmax of the input image dataD1 by the coefficient Ks), the parameters RA, RB, WBR, and WRZ, thetentative distribution ratio, and the coefficient Ks, for each pixel.Thus, according to the image display device according to the embodiment,the more preferable distribution ratio WRs depending on the coefficientKs is obtained, and thus it is possible to suppress the occurrence ofcolor breakup while image display having high color reproduction isperformed.

The above-described effects in the embodiment will be specificallydescribed with reference to FIGS. 12 and 13. FIG. 12 is a diagramillustrating an operation of a field sequential image display device inthe related art (referred to as “a conventional example” below), inwhich the distribution ratio WRs of a white subframe as a common colorsubframe is fixed to 100%. The brightness (input brightness) of inputimage data in an operation example illustrated in FIG. 12(B) is smallerthan an input brightness in an operation example illustrated in FIG.12(A). A curved line illustrated by a bold dotted line for driving imagedata in FIG. 12 schematically indicates a response of a liquid crystalpanel as a display device (similar in FIG. 13 described later). Asunderstood from FIG. 12, in the conventional example, a difference inhue, saturation, and luminance occurs between an extended input color(color indicated by image data after amplification and compressionprocessing) and an actual display color (color which is actuallydisplayed in the display device), by the influence of the responsecharacteristics of the display device. Thus, image display havingsufficiently high color reproduction is not performed. On the contrary,the image display device 1 according to the embodiment operates on inputimage data which is the same as the input image data in the operationexample in FIG. 12(A), as illustrated in FIG. 13(A). The image displaydevice 1 operates on input image data which is the same as the inputimage data in the operation example in FIG. 12(B), as illustrated inFIG. 13(B). As described above, in the embodiment, in a case wherecolor-breakup handling processing is performed (case of WRs=WRsv2), thedistribution ratio WRs increases as the adjusted brightness V becomessmaller. In an operation example in FIG. 13(A), WRs is 66%. In anoperation example in FIG. 13(B), WRs is 90%. In a case where theadjusted brightness V is small, a gradation difference between subframes(difference between WBGR pixel data values) is small. Thus, an influenceof the response characteristics of the display device is small.Therefore, according to the embodiment, since a difference in any ofhue, saturation, and luminance between the extended input color and theactual display color is reduced, it is possible to suppress theoccurrence of color breakup while image display having high colorreproduction is performed.

In the embodiment, in Step S106, the driving image-data operation unit33 obtains image data Wd, Bd, Gd, and Rd of the four colors by theexpressions (5a) to (5d), based on the image data Ri, Gi, and Bi of thethree colors, the minimum value Dmin, the distribution ratio WRs, theadjustment coefficient Ks, and the parameter WBR. Here, a color shown bythe image data Ri, Gi, or Bi of the three colors is referred to as “acolor before conversion”, and a color shown by the image data Wd, Bd,Gd, or Rd of the four colors is referred to as “colors afterconversion”. When the two colors are expressed in an HSV color space,brightness V is different between the two colors, the hue H is the samebetween the two colors, and the saturation S is the same between the twocolors. As described above, in image-data conversion processing in theimage data conversion unit 10, for each pixel, the hue H holds the samevalue and the saturation S holds the same value in the HSV color space,between the input image data D1 and the driving image data D2.

The image display device 1 according to the embodiment obtains thedistribution ratio WRs and the coefficient Ks based on the saturation Sand the parameter WRX and obtains the driving image data D2 with thedistribution ratio WRs and the coefficient Ks which have been obtained.Thus, according to the image display device 1, since the preferableparameter WRX depending on the response characteristics and the like ofthe display unit 20 is set, and the gradation is displayed at a fasterresponse rate, it is possible to improve color reproduction.

In the embodiment, in the distribution ratio-and-coefficient computationunit 13, for each pixel, the first distribution ratio WRs11 is obtainedbased on the saturation S and the parameter WRX such that driving imagedata (white image data Wd) corresponding to the common color subframe isin a range from the minimum value Ddmin of driving image data (blueimage data Bd, green image data Gd, and red image data Rd) correspondingto other subframes to the maximum value Ddmax thereof. The distributionratio WRs of the white subframe is determined based on the obtainedfirst distribution ratio WRs1. Thus, it is possible to suppress thechange of the luminance of the pixel 26 in one frame period and toimprove color reproduction of the image display device. The image dataconversion unit 10 can obtain the distribution ratio WRs and theadjustment coefficient Ks by the functions which smoothly changedepending on the saturation S (see FIGS. 9 and 10). Thus, it is possibleto prevent distortion of an image when a gradation image is displayed.

In the embodiment, the distribution ratio-and-coefficient computationunit 13 obtains the maximum value allowed to be taken by the coefficientKs, based on the saturation S and the parameter WRX, as the coefficientKs. The distribution ratio-and-coefficient computation unit obtains themaximum value under a condition in which the distribution ratio WRs isused, and the maximum value Dmax of input image data D1 is set toMaximum Value 1 allowed to be taken by the input image data D1, for eachpixel. Thus, it is possible to obtain a large coefficient Ks in anallowable range and to perform amplification and compression on theinput image data D1 in an allowable range.

In the embodiment, in the conversion processing by the image dataconversion unit 10, the range of the maximum value DDmax of second imagedata in one frame period is determined in accordance with the minimumvalue DDmin of the second image data in one frame period (see theexpression (1) and FIG. 2). Thus, it is possible to suppress a change ofthe image data after the conversion, in one frame period, and to improvecolor reproduction of the image display device.

In the image display device 1 according to the embodiment, the parameterstorage unit 11 stores the parameter WBR as a third parameter fordesignating the luminance of the light source 27 provided in the displayunit 20 when the common color subframe (white subframe) is displayed, inaddition to the parameter WRX as the first parameter and the parametersRA and RB as the second parameter. The display unit 20 controls theluminance of the light source 27 in accordance with the third parameter,when displaying the common color subframe. Accordingly, according to theembodiment, the preferable parameter WRX is set in accordance with theresponse characteristics of the display unit 20, and the maximum valueDDmax of driving image data D2 in one frame period is limited by usingthe second parameter, in accordance with the minimum value DDmin of thedriving image data D2 in the one frame period (see FIG. 2). Thus, it ispossible to more improve color reproduction. In addition, the luminanceof the light source 27 when the common color subframe is displayed iscontrolled with the third parameter (parameter WBR), and thus it ispossible to reduce heat generated by the light source 27.

In the embodiment, the image data conversion unit 30 performs theconversion processing on normalized luminance data (input image dataD1). Thus, it is possible to accurately perform the conversionprocessing. The input image data D1 corresponds to the red, green, andblue colors. The driving image data D2 corresponds to red, green, blue,and white subframes. The common color subframe is a white subframe.Thus, in the image display device that displays subframes of threeprimary colors and the white color based on input image data D1corresponding to the three primary colors, it is possible to suppressthe occurrence of noise at a low-luminance portion of a display imagewhile the gradation properties are held.

<1.8 Other Modification Examples of First Embodiment>

In Step S104, the distribution ratio-and-coefficient computation unit 13may obtain the first distribution ratio WRs1 and the coefficient Ks bycalculation expressions other than the expressions (18a) to (18c) and(21). In Step S201, the distribution ratio-and-coefficient computationunit 13 may obtain the second distribution ratio WRsv2 by anothercalculation expression satisfying the expression (28). In the abovedescriptions for the embodiment, the first distribution ratio WRs1 isgiven by the expression (17) or the expressions (18a) to (18c), and thecoefficient Ks is given by the expression (21). However, these are notlimited thereto. For example, a configuration (referred to as “a thirdmodification example” below) in which the first distribution ratio WRs1and the coefficient Ks are given by the following expressions (31) and(32a) may be made. A configuration (referred to as “a fourthmodification example” below) in which the first distribution ratio WRs1and the coefficient Ks are given by the following expressions (31) and(32b) may be made.

WRs1=min(WBRo/(1−S),WRX)  (31)

Ks=1/[1−WRs1(1−S)]  (32a)

Ks=RB/[1−{WRs1(1−RA)+RA}(1−S)]  (32b)

In the expression (31), WBRo is ½.

In the image display device according to the third modification example,the distribution ratio-and-coefficient computation unit obtains thesecond distribution ratio WRsv2 by the expressions (30a) to (30c), forexample. In the expressions (30a) to (30c), WBRb=1 and V=Dmax·Ks areestablished, and Tsv is given by the following expression (33). WRsv2 isset to WRs1 at time of WRZ≤WRs1.

Tsv=3·Ks/[2{Ks(1−S)(WRZ−WRs1)+1}]  (33)

In the image display device according to the third modification example,the distribution ratio-and-coefficient computation unit may obtain thesecond distribution ratio WRsv2 by another calculation expressionsatisfying the expression (28) when RA=0 and RB=WBR=1 are set in theexpressions (27a) to (27c).

In the image display device according to the fourth modificationexample, the distribution ratio-and-coefficient computation unit obtainsthe second distribution ratio WRsv2 by the expressions (30a) to (30c),for example. Tsv is given the expressions (29), and WBRb=RB/(1+RA) andV=Dmax·Ks are established in the expression (29) and the expressions(30a) to (30c). The parameter WRZ takes a value in a range of WRs≤WRZ≤1.In the image display device according to the fourth modificationexample, the distribution ratio-and-coefficient computation unit mayobtain the second distribution ratio WRsv2 by another calculationexpression satisfying the expression (28) when WBR=1 is set in theexpressions (27a) to (27c).

2. Second Embodiment

<2.1 Overall Configuration>

FIG. 14 is a block diagram illustrating a configuration of an imagedisplay device according to a second embodiment. An image display device3 illustrated in FIG. 14 includes an image data conversion unit 30 and adisplay unit 40. The image data conversion unit 30 is obtained in amanner that, in the image data conversion unit 30 in the firstembodiment, the distribution ratio-and-coefficient computation unit 13is replaced with a distribution ratio-and-coefficient computation unit32, and the parameter storage unit 11 is replaced with a parameterstorage unit 31. Differences from the first embodiment will be describedbelow.

The image display device according to the embodiment selectivelyperforms low-luminance-portion noise handling processing and furtherselectively performs high-luminance-portion noise handling processing.In the image display device according to the embodiment, the parameterstorage unit 31 stores parameters NR, GL, RC, WRY, WRZ0, and WRZ1 inaddition to the parameters WRX, RA, RB, and WBR. The distributionratio-and-coefficient computation unit 32 obtains the coefficient Ks bya calculation expression different from that in the first embodiment,when high-luminance-portion noise handling processing is performed.

The parameter GL indicates the type of high-luminance-portion noisehandling processing and takes a value of 0, 1, or 2. The value of 0indicates that high-luminance-portion noise handling processing is notperformed. The value of 1 or 2 indicates that the high-luminance-portionnoise handling processing is performed. The parameter RC is provided ina calculation expression of obtaining the coefficient Ks when thehigh-luminance-portion noise handling processing is performed. Theparameter RC takes a value in a range of 0≤RC<1.

FIG. 15 is a flowchart illustrating image-data conversion processingaccording to the embodiment. The flowchart illustrated in FIG. 15 isobtained by adding Steps S300 to S304 and S106 to S109 to the flowchartillustrated in FIG. 3. The image data conversion unit 30 operatessimilar to the first embodiment, in Steps S101 to S104.

Then, the distribution ratio-and-coefficient computation unit 32performs condition branching in accordance with the parameter GL (StepS300). The distribution ratio-and-coefficient computation unit 32 causesthe process to proceed to Step S105 at time of GL=0, and to proceed toStep S301 at time of GL>0. In the former case, the distributionratio-and-coefficient computation unit 32 obtains the coefficient Ks bythe following expression (40) (Step S105) (see the expression (21)).

Ks=RB/[1−{WRs1(1−RA)+RA}(1−S)]  (40)

In the latter case, the distribution ratio-and-coefficient computationunit 32 sets the parameters RA and RB to satisfy RA=0 and RB=1 (StepS301) and obtains a tentative coefficient Ks′ by the followingexpression (41a) (Step S302). Then, the distributionratio-and-coefficient computation unit 32 obtains a correctioncoefficient Kh by the following expression (41b) at time of GL=1, andobtains the correction coefficient Kh by the following expression (41c)at time of GL=2 (Step S303). The correction coefficient Kh increases asthe saturation S becomes smaller. The distribution ratio-and-coefficientcomputation unit 32 outputs a result obtained by multiplying thetentative coefficient Ks′ by the correction coefficient Kh, as theadjustment coefficient Ks (Step S304).

Ks′=1/{1−WRs1(1−S)}  (41a)

Kh=1−RC·S  (41b)

Kh=1−RC·S ²  (41c)

Then, the distribution ratio-and-coefficient computation unit 32performs condition branching in accordance with the parameter NR (StepS106). The distribution ratio-and-coefficient computation unit 32 causesthe process to proceed to Step S201 at time of NR=0, and to proceed toStep S107 at time of NR=1. In the latter case, the distributionratio-and-coefficient computation unit 32 obtains a value NS based onthe coefficient Ks and the parameter WBR (Step S107), obtains acoefficient Ksv based on the maximum value Dmax indicating the inputbrightness Vi, the coefficient Ks, and the value NS (Step S108), andsets the coefficient Ksv as the coefficient Ks (Step S109).

Then, the distribution ratio-and-coefficient computation unit 32 obtainsthe second distribution ratio WRsv2 by a calculation expression (whichwill be described later) based on the saturation S, the adjustedbrightness V, the parameters WRX, WBR, WRZ0, WRZ1, and WRY, the firstdistribution ratio WRs1, and the coefficient Ks (Step S201).

Then, the distribution ratio-and-coefficient computation unit 32performs condition branching in accordance with the parameter VCBU (StepS202). Similar to the first embodiment, the parameter VCBU indicateswhether or not color-breakup handling processing is performed, and takesa value of 0 or 1. The value of 0 indicates that the color-breakuphandling processing is not performed. The value of 1 indicates that thecolor-breakup handling processing is performed. The distributionratio-and-coefficient computation unit 32 sets the first distributionratio WRs1 as the distribution ratio WRs of the white subframe at timeof VCBU=0 (Step S203) and sets the second distribution ratio WRsv2 asthe distribution ratio WRs of the white subframe at time of VCBU=1 (StepS204).

The driving image-data operation unit 33 obtains image data Wd, Bd, Gd,and Rd of four colors based on the image data Ri, Gi, and Bi of thethree colors, the minimum value Dmin, the distribution ratio WRs, thecoefficient Ks, and the parameter WBR by the following expressions (42a)to (42d) (Step S205).

Wd=WRs·Dmin·Ks·PP/WBR  (42a)

Bd=(Bi−WRs·Dmin)Ks·PP  (42b)

Gd=(Gi−WRs·Dmin)Ks·PP  (42c)

Rd=(Ri−WRs·Dmin)Ks·PP  (42d)

Here, in the expressions (42a) to (42d), PP indicates a value (=P/Pmax)obtained by dividing the maximum value P for image data constraint bythe maximum value Pmax (=1) which may be set for the image data. PP isalso used in a gradation compression method in which the saturation S isnot considered. In the following descriptions, PP=1 is assumed. In acase of PP≠1, outputting the maximum luminance when S is 0 is notpossible.

The driving image-data operation unit 33 obtains image data Wd, Bd, Gd,and Rd of four colors by using the coefficient Ks obtained in Step S105or S304 when NR is 0, and obtains the image data Wd, Bd, Gd, and Rd ofthe four colors by using the coefficient Ksv obtained in Step S108 whenNR is 1. As described above, the image data conversion unit 10 does notperform low-luminance-portion noise handling processing when NR is 0,and performs low-luminance-portion noise handling processing when NR is1 (details will be described later).

<2.2 Regarding Adjustment Coefficient Ks>

As illustrated in FIG. 4 and described above, the range of (S, WRs),which is indicated by the saturation S and the distribution ratio WRs isdivided into the first area in which Ddmin<Wd<Ddmax is satisfied, thesecond area in which Ddmax<Wd is satisfied, and the third area in whichWd<Ddmin is satisfied. In a case where the distribution ratio WRs isdetermined to cause (S, WRs) to be in the first area, the expression ofDdmin<Wd<Ddmax is established, and a difference between image data Wd,Bd, Gd, and Rd of four colors included in the driving image data D2becomes the minimum ((Ddmax−Ddmin) is established). It is desirable that(S, WRs) is in the first area, so long as color breakup does not occur.Therefore, in the embodiment, in a case of GL>0 (case wherehigh-luminance-portion noise handling processing is performed), thecoefficient Ks=1/{1−WRs(1−S)} obtained by substituting RA=0 and RB=1into the expression (40) is set as the tentative coefficient Ks′ (seethe expression (41a)), and a result obtained by multiplying thetentative coefficient Ks′ by the correction coefficient Kh (expressions(41b) and (41c)) is set as the adjustment coefficient Ks.

FIG. 16 is a diagram illustrating a graph for the adjustment coefficientKs in the image display device according to the embodiment. Theadjustment coefficient Ks in a case of GL=0 is given by the expression(40) including the parameters RA and RB (Step S105) and is indicated bya curved line of a bold one-dot chain line in FIG. 16. The curved lineof the bold one-dot chain line indicates the adjustment coefficient Kswhen the parameters RA and RB are set to be RA=0.25 and RB=0.75. On thecontrary, in a case of GL>0 (here, GL is set to 2), the parameters RAand RB are set to be RA=0 and RB=1 (Step S301). The adjustmentcoefficient Ks is given by an expression of Ks=Kh·Ks′ based on theexpressions (41a) and (41c) and is indicated by a curved line of a boldsolid line in FIG. 16. That is, the expression (41a) corresponding tothe expression of Ks=1/{1−WRs(1−S)} obtained by substituting RA=0 andRB=1 into the expression (40) represents a curved line of a bold brokenline in FIG. 16. The expression of Ks=Kh·Ks′ representing a curved lineof a bold solid line is obtained by multiplying the expression (41a) bythe expression (41c), that is, Kh=1-RC·S2. The curved line of the boldsolid line represents the adjustment coefficient Ks when the parameterRC is set to 0.25. The function Ks=RB/[1−{WRs(1−RA)+RA}(1−S)]represented by the expression (40) is referred to as “a first function”below. The function Ks=Kh·Ks′(1−RC·S2)/{1−WRs(1−S)} obtained based onthe expressions (41a) and (41c) is referred to as “a second function”below.

The distribution ratio-and-coefficient computation unit 32 obtains thecoefficient Ks with the first function at time of GL=0 (Step S105). Thedistribution ratio-and-coefficient computation unit obtains thecoefficient Ks with the second function at time of GL=2 (Steps S302 toS304). The second function is defined by using the functionKs=1/{1−WRs(1−S)} as auxiliary. The second function is expressed asKs=Kh/{1−WRs(1−S)} (Kh is a function based on the saturation S) andtakes the same value as that in the first function at time of S=0. Thesecond function preferably takes the same value RB as that in the firstfunction at time of S=1.

In a case where the coefficient Ks is obtained with the first function(case of GL=0), when the saturation S is small and the luminance ishigh, the coefficient Ks largely changes with respect to the change ofthe saturation S (see an H1 portion in FIG. 16). Therefore, in a casewhere the coefficient Ks is obtained with the first function, gradationskipping may occur in a display image, and noise may occur at ahigh-luminance portion included in the display image. On the contrary,in a case where the coefficient Ks is obtained with the second function(case of GL>0), the amount of the coefficient Ks changing with respectto the change of the saturation S is small even though the saturation Sis small, and the luminance is high (see an H2 portion in FIG. 16).Thus, if the coefficient Ks is obtained with the second function, it ispossible to suppress the occurrence of gradation skipping in a displayimage and to suppress an occurrence of noise at a high-luminance portionincluded in the display image.

As described above, in the image display device according to theembodiment, at time of GL>0, for each pixel, the distributionratio-and-coefficient computation unit 32 obtains the first distributionratio WRs1, the tentative coefficient Ks′, and the correctioncoefficient Kh (which becomes smaller as the saturation S increases)based on the saturation S and the parameters WRX, WBR, and RC(expressions (41b) and (41c)). The distribution ratio-and-coefficientcomputation unit 32 outputs a result obtained by multiplying thetentative coefficient Ks′ by the correction coefficient Kh, as thecoefficient Ks. Thus, according to the image display device according tothe embodiment, it is possible to suppress the occurrence of noise at ahigh-luminance portion of a display image. A specific method ofobtaining the first distribution ratio WRs1 will be described later.

<2.3 Method of Determining Function of Obtaining First DistributionRatio WRs1>

The distribution ratio-and-coefficient computation unit 32 has afunction of obtaining the first distribution ratio WRs1 based on thesaturation S and a second function of obtaining the adjustmentcoefficient Ks based on the saturation S at time of NR=0. The functionschange depending on the parameters RA, RB, WRX, and WBR stored in theparameter storage unit 31, in a case of GL=0. The functions changedepending on the parameters WRX, WBR, and RC stored in the parameterstorage unit 31, in a case of GL>0. Details of the former function, thatis, the method of determining the function of obtaining the firstdistribution ratio WRs1 are similar to those in the first embodiment(see the expressions (17), (18a) to (18c), and the like). Details of thelatter function are as described above for the adjustment coefficientKs. As described above, the function of obtaining the coefficient Ks isrepresented with the first distribution ratio WRs1 (see the expressions(40) and (41a)). However, since the first distribution ratio WRs1 isobtained based on the saturation S, the function of obtaining thecoefficient Ks is a function based on the saturation S. In a firstexample and a second example described below, the parameter WRX takes avalue in a range of ½≤WRX≤1.

<2.4 Case where Low-Luminance-Portion Noise Handling Processing isPerformed>

Next, a method of determining the function of obtaining the adjustmentcoefficient Ks in a case where low-luminance-portion noise handlingprocessing is performed (case of NR=1) will be described (see Steps S107to S109 in FIG. 15).

When NR is 1, the distribution ratio-and-coefficient computation unit 13obtains the value NS by the following expression (43) in Step S107 andobtains the coefficient Ksv by the following expression (44) in StepS108.

NS=NB−NB{Ks−(1+WBR)}2/(1+WBR)²  (43)

Ksv=(Ks−NS)Vi+NS  (44)

NB in the expression (43) is given by the following expression.

NB=(1+WBR)²/{2(1+WBR)−1}  (45)

Vi indicates the input brightness and is given by the followingexpression.

Vi=Dmax=max(Ri,Gi,Bi)  (46)

If the expression (43) is substituted into the expression (44), acalculation expression (referred to as “Expression E” below) ofobtaining the coefficient Ksv based on the input brightness Vi, thecoefficient Ks, and the parameter WBR is derived. If Vi is set to 0 inExpression E, the function of obtaining the coefficient Ksv when Vi is 0is obtained. Similarly, if Vi is set to 1 in Expression E, the functionof obtaining the coefficient Ksv when V is 1 is obtained. If Vi is setto Vx (here, 0<Vx<1) in Expression E, the function of obtaining thecoefficient Ksv when Vi is Vx is derived. The coefficient Ksv at time ofVi=0 is equal to the value NS (Ksv=NS), and the coefficient Ksv at timeof Vi=1 is equal to the coefficient Ks (Ksv=Ks). The coefficient Ksv attime of Vi=Vx has a value obtained by dividing the coefficient Ks andthe value NS at a ratio of (1−Vx):Vx.

FIG. 17 is a diagram illustrating a graph of the coefficient Ksv. FIGS.17(A) to 17(C) illustrate graphs at time of Vi=0, Vi=Vx, and Vi=1,respectively. As illustrated in FIG. 17, when the brightness V is set toa certain value, the coefficient Ksv decreases as the saturation Sbecomes greater, regardless of the value of the brightness Vi.Therefore, the coefficient Ksv becomes the maximum at time of S=0, andbecomes the minimum at time of S=1. The difference between the minimumvalue and the maximum value of the coefficient Ksv at time of Vi=0 issmaller than the difference between the minimum value and the maximumvalue of the coefficient Ksv at time of Vi=1. The difference between theminimum value and the maximum value of the coefficient Ksv decrease asthe brightness Vi becomes smaller.

As described above, since the difference between the minimum value andthe maximum value of the coefficient Ksv decreases as the brightness Vibecomes smaller, the amount of the coefficient Ksv changing with respectto the amount of the saturation S changing is small when the brightnessVi is small. Thus, if low-luminance-portion noise handling processing isperformed, it is possible to suppress an occurrence of a situation inwhich the color of a pixel largely changes between the pixel and theadjacent pixel when the luminance is low, and to suppress the occurrenceof noise at a low-luminance portion of a display image.

In the image display device 3, if the saturation S and the hue H are thesame, it is necessary that the luminance of a pixel 26 increases as theinput image data D1 becomes greater (that is, gradation properties areheld). In order to hold the gradation properties, if the saturation S isthe same, it is necessary that a result obtained by performingamplification and compression processing on the brightness V increasesas the brightness Vi of the input image data D1 becomes greater. Thus,at least, it is necessary that a result obtained by multiplying thebrightness Vi by the coefficient Ksv at time of 0<Vi<1 is smaller than aresult obtained by multiplying the brightness Vi (=1) by the coefficientKsv (=Ks) at time of Vi=1. With Ksv·Vi≤Ks, the following expression (47)is obtained.

Ksv≤Ks/Vi  (47)

A range satisfying the expression (47) corresponds to a shaded areaillustrated in FIG. 18. The function of obtaining the coefficient Ksvbased on the brightness Vi is determined such that the graph of thefunction is in the shaded area illustrated in FIG. 18. As describedabove, the distribution ratio-and-coefficient computation unit 32obtains the coefficient Ksv by the expression (44). As illustrated inFIG. 18, the graph of the coefficient Ksv passes through two points (0,NS) and (1, Ks).

In order to cause an in equation obtained by substituting the expression(44) with the expression (47) to be established in a range of 0<Vi<1,the slope of a straight line shown by the expression (44) may be equalto or greater than the slope of a tangent line at a point (1, Ks) of thefunction of Ksv=Ks/Vi. Thus, with Ks−NS≥−Ks, the following expression(48) is derived. A range satisfying the expression (48) corresponds to adot pattern area illustrated in FIG. 19.

NS≤2Ks  (48)

FIG. 20 is a diagram illustrating a graph of the value NS. The graphillustrated in FIG. 20 passes through three points (0, 0), (1, 1), and(1+WBR, NB). The slope of a tangent line at a point (0, 0) of thefunction of obtaining the value NS satisfies the expression of2NB/(1+WBR)=(2+2WBR)/(1+2WBR), and is equal to or smaller than 2 in arange of 0≤WBR≤1. Thus, the graph illustrated in FIG. 20 is in the rangeillustrated in FIG. 19. Accordingly, since the value NS is obtained bythe expression (44), if the saturation S and the hue H are the same, theresult obtained by performing amplification and compression processingon the brightness Vi increases as the brightness Vi of the input imagedata D1 becomes greater. Thus, in a case where low-luminance-portionnoise handling processing is performed, the luminance of a pixel 26increases as the input image data D1 becomes greater, and thus it ispossible to hold the gradation properties.

The effects of low-luminance-portion noise handling processing will bedescribed with reference to FIGS. 21 to 23. FIG. 21 is a diagramillustrating a graph of the coefficient in the image display device 3.FIG. 21 illustrates the graph of the coefficient Ks obtained in StepS105 at time of NR=0 and the graph of the coefficient Ksv obtained inStep S108 at time of NR=1. Here, WRX=WBR=1 and RA=RB=0.5 are set. FIG.22 is a diagram illustrating an example of image-data conversionprocessing in a case where low-luminance-portion noise handlingprocessing is not performed (at time of NR=0), in the image displaydevice 3. FIG. 23 is a diagram illustrating an example of image-dataconversion processing in a case where the low-luminance-portion noisehandling processing is performed (at time of NR=1), in the image displaydevice 3.

Here, as an example, a case where red image data, green image data, andblue image data which are included in input image data D1 corresponds to(0.25, 0.25, 0.25) and a case where the red image data, green imagedata, and blue image data corresponds to (0.25, 0.25, 0.2) areconsidered (the former is referred to as “data Da” below, and the latteris referred to as “data Db” below). Regarding data Da, S is 0, and V is0.25. Regarding data Db, S is 0.2, and Vi is 0.25.

When NR is 0, and S is 0, Ks is 2. When NR is 0, and S is 0.2, Ks is1.428 (see FIG. 21). Thus, in a case where low-luminance-portion noisehandling processing is not performed (FIG. 22), amplification andcompression processing of multiplying the data Da by Ks=2 is performed,and image data after the amplification and compression processingcorresponds to (0.5, 0.5, 0.5). Amplification and compression processingof multiplying the data Db by Ks=1.428 is performed, and image dataafter the amplification and compression processing corresponds to(0.357, 0.357, 0.286). A difference between the data Da and the data Dbis small. However, in a case where the low-luminance-portion noisehandling processing is not performed, a large difference occurs betweena result obtained by performing amplification and compression processingon the data Da and a result obtained by performing amplification andcompression processing on the data Db.

When NR is 1, and S is 0, Ks is 1.333. When NR is 1, and S is 0.2, Ks is1.224 (see FIG. 21). Thus, in a case where low-luminance-portion noisehandling processing is performed (FIG. 23), amplification andcompression processing of multiplying the data Da by Ks=1.333 isperformed, and image data after the amplification and compressionprocessing corresponds to (0.333, 0.333, 0.333). Amplification andcompression processing of multiplying the data Db by Ks=1.224 isperformed, and image data after the amplification and compressionprocessing corresponds to (0.306, 0.306, 0.245). In a case where thelow-luminance-portion noise handling processing is performed, thedifference between the result obtained by performing amplification andcompression processing on the data Da and the result obtained byperforming amplification and compression processing on the data Db issmaller than that in a case where the low-luminance-portion noisehandling processing is not performed.

It is assumed that a pixel driven based on the data Da is adjacent to apixel driven based on the data Db. In a case where thelow-luminance-portion noise handling processing is not performed, thedifference of the color between the two pixels is large, and thus noiseoccurs at a low-luminance portion of a display image. Since thelow-luminance-portion noise handling processing is performed, thedifference of the color between the two pixels is reduced, and thus itis possible to suppress the occurrence of noise at the low-luminanceportion of the display image.

<2.5 Method of Determining Function of Obtaining Second DistributionRatio WRsv2>

As described above, in a case of VCBU=0, that is, a case wherecolor-breakup handling processing is not performed, the firstdistribution ratio WRs1 is set as the distribution ratio WRs of thewhite subframe. In a case of VCBU=1, that is, a case where thecolor-breakup handling processing is performed, the second distributionratio WRsv2 is set as the distribution ratio WRs of the white subframe(Steps S202 to S204). The function of obtaining the first distributionratio WRs11 is determined in the above-described manner. A method ofdetermining the function of obtaining the second distribution ratioWRsv2 will be described below. In a case of GL=0 (case wherehigh-luminance-portion noise handling processing is not performed), themethod of determining the function of obtaining the second distributionratio WRSv2 is similar to that in the first embodiment (see theexpressions (27a) to (30c) and FIG. 11). The method of determining thefunction of obtaining the second distribution ratio WRSv2 in a case ofGL>0 (case where high-luminance-portion noise handling processing isperformed) will be described later.

The embodiment is different from the first embodiment in that, in a caseof GL>0, RA=0 and RB=1 in the expression (1) are set, and the minimumvalue DDmin and the maximum value DDmax of driving image data D2 in oneframe period are not directly limited by the values of the parameters RAand RB. The minimum value Rsva of the second distribution ratio WRsv2 ina case where (S, WRs1) is in the first area, the maximum value Rsvb ofthe second distribution ratio WRsv2 in a case where (S, WRs1) is in thesecond area, and the minimum value Rsvc of the second distribution ratioWRsv2 in a case where (S, WRs1) is in the third area are given bysubstituting RA=0 and RB=1 into the expressions (27a) to (27c), as inthe following expression (Ea) to (Ec) (see FIGS. 4 and 11).

WRsva=(V−1)/{V(1−S)}  (Ea)

WRsvb=WBR/{V(1−S)}  (Eb)

WRsvc=(V−1)/{V(1−S)}  (Ec)

FIG. 24 is a diagram illustrating a method of determining a function ofobtaining WRsv2. A curved line illustrated by a one-dot chain line inFIG. 24 indicates the second distribution ratio WRsv2 at time of S=0.5in the embodiment. Here, RA=0, RB=1, RC=0.25, WBR=0.5, WRX=0.75,WRY=0.85, WRZ0=0.6, and WRZ1=1 are set. The distributionratio-and-coefficient computation unit 32 obtains the first distributionratio WRs1 and the coefficient Ks by the calculation expressions(expressions (18a) to (18c) and (21)) according to the secondmodification example of the first embodiment. In FIG. 24, a portionsurrounded by a curved line of a broken line, which is indicated by Ca,Cc, a curved line of a broken line, which is indicated by Cb, a straightline of WRsv2=1, a straight line of WRsv2=0, and a straight line of V=0corresponds to a range of (V, WRsv2) represented by the adjustedbrightness (brightness after amplification and compression processing) Vand the second distribution ratio WRsv2 satisfying the expression (28)(this range is referred to as “a distribution-ratio allowable range”below). The function of obtaining the second distribution ratio WRsv2 isdetermined such that a curved line indicating the function is in thedistribution-ratio allowable range.

In the first embodiment, a function indicated by a curved line passingthrough a point (Ks, WRs1) in a V-WRsv2 plane (plane defined by a V axisand a WRsv2 axis perpendicular to each other) is defined as the functionof obtaining the second distribution ratio WRsv2 (see FIG. 11 and theexpressions (30a) to (30c)). However, the curved line indicating thesecond distribution ratio WRsv2 may be in the above-describeddistribution-ratio allowable range in FIG. 24. In the embodiment, afunction indicated by a curved line passing through a point (Ks, WRs3)is defined as the function of obtaining the second distribution ratioWRsv2.

Here, WRsβ is defined as WRs1 when WBRo=WBR/Ks and WRX=WRY are set inthe expressions (18a) to (18c). As illustrated in FIG. 25, WRsβ definedas described above is a smooth function of the saturation S, which doesnot have an inflection point in a settable range (function whichsmoothly changes with respect to the change of the saturation S). InFIG. 24, an intersection point between the straight line of V=Ks and thecurved line Cb corresponds to the settable maximum value of WRsβ, and anintersection point between the straight line of V=Ks and the curved lineCa, Cc corresponds to the settable minimum value of WRsβ. In FIG. 25, acurved line of a one-dot chain line and a curved line of a two-dot chainline indicate functions of the saturation S, which represent thesettable maximum value and the settable minimum value of WRsβ,respectively.

WRX≤WRY≤1 is desirable, and the occurrence of color breakup can besuppressed by satisfying this inequality. If WRX and WRY are adjusted,it is possible to set a gradation (driving WBGR pixel data values) ineach subframe as a respondable range of the display device (liquidcrystal panel 24), and to suppress an occurrence of breakdown of animage.

In the first embodiment, WRZ is set to a fixed value. However, WRZ isdesirably set to be allowed to be adjusted in accordance with a responserange of the display device (liquid crystal panel 24). In theembodiment, if it is considered that WRsβ is set as the firstdistribution ratio WRs1, and the expressions (29) and (30a) to (30c) ofgiving the second distribution ratio WRsv2 are considered, the functionof obtaining WRZ may be set in a range of WRsβ≤WRZ≤1 in order to setWRsv2 to increase as the adjusted brightness V becomes smaller.

For example, WRZ can be defined by the following expression with theparameters WRZ0 and WRZ1.

WRZ=WRsβ+(1−WRsβ)WRZs  (En)

WRZs in the expression (En) is defined by any of the followingexpressions (EE1) to (EE3).

WRZs=(WRZa−WRZb)S+WRZb  (EE1)

WRZs=(WRZa−WRZb)S ² +WRZb  (EE2)

WRZs=(WRZb−WRZa)(1−S)² +WRZa  (EE3)

In the expressions (EE1) to (EE3), WRZa and WRZb are defined by thefollowing expressions.

WRZa=(WRZ1−WRY)/(1−WRY)  (50a)

WRZb=(WRZ0−WBR/Ks)/(1−WBR/Ks)  (50b)

WRY≤WRZ1  (50c)

WBRo≤WRZ0  (50d)

WRZ defined by the following expression may be used instead of WRZdefined by the expression (En) and the expression (EEi) (i=1, 2, 3).

WRZ=WRZ1−(WRZ1−WRZ0)(1−S)³  (E4)

In FIG. 25, a curved line indicated by Ci represents a graph of WRs=WRZin a case where WRZ is defined by the expression (En) and the expression(EEi) (i=1, 2, 3). A curved line indicated by C4 represents a graph ofWRs=WRZ in a case where WRZ is defined by the expression (E4). Afunction of obtaining WRZ is not limited to a function defined by theexpression (En) and the expression (EEi) (i=1, 2, 3) or a functiondefined by the expression (E4). Any function may be provided as thefunction of obtaining WRZ so long as the function is properly set in arange of WRsβ≤WRZ≤1 with respect to the response characteristics of thedisplay device (liquid crystal panel 24).

In the embodiment, the function of obtaining the second distributionratio WRsv2 is defined by the function obtained by substitutingWRs1=WRsβ, RA=0, and RB=1 into the expressions (29) and (30a) to (30c)on the assumption that WRZ is defined with the parameters WRZ0, WRZ1,and WRY as described above.

Since the function of obtaining the second distribution ratio WRsv2 isdefined in this manner, it is possible to set a gradation in eachsubframe in a respondable range of the display device. The seconddistribution ratio WRsv2 can be adjusted to a very large value in asettable range without providing an inflection point. Thus, it ispossible to reduce the occurrence of color breakup more.

<2.6 Effects of Second Embodiment>

As described above, in the image display device according to theembodiment, at time of VCBU=1, the distribution ratio-and-coefficientcomputation unit 32 obtains the tentative distribution ratio (firstdistribution ratio WRs1) and the coefficient Ks based on the saturationS and the parameters WRX and WBR and obtains the distribution ratio WRs(second distribution ratio WRsv2) increasing as the adjusted brightnessV becomes smaller, based on the adjusted brightness V (brightness afteramplification and compression processing) (obtained by multiplying themaximum value Dmax of the input image data D1 by the coefficient Ks),the parameters WBR and WRZ, the tentative distribution ratio, and thecoefficient Ks, for each pixel. In a case of GL>0, the seconddistribution ratio WRsv2 is defined by the expression obtained bysubstituting WRs1=WRsβ, RA=0, and RB=1 into the expressions (18a) to(18c) representing the second distribution ratio WRsv2 in the firstembodiment. The parameter WRZ does not have a fixed value and isappropriately set with the parameters WRZ0 and WRZ1 in a range ofWRsβ≤WRZ≤1. Therefore, the second distribution ratio WRsv2 can beadjusted so as to be very large in the settable range while thecharacteristics in which the second distribution ratio WRsv2 increasesas the adjusted brightness V becomes smaller are provided (see FIGS. 24and 25). Thus, it is possible to suppress the occurrence of colorbreakup more while image display having high color reproduction isperformed (see FIGS. 12 and 13), in comparison to the technology in therelated art.

In the embodiment, in Step S205, the driving image-data operation unit33 also obtains image data Wd, Bd, Gd, and Rd of the four colors by theexpressions (42a) to (42d), based on the image data Ri, Gi, and Bi ofthe three colors, the minimum value Dmin, the distribution ratio WRs,the adjustment coefficient Ks, and the parameter WBR. Here, when colors(colors before conversion) represented by image data Ri, Gi, and Bi ofthe three colors and colors (colors after conversion) represented byimage data Wd, Bd, Gd, and Rd of the four colors are expressed in an HSVcolor space, the brightness V differs between two colors, but the hue Hand the saturation S have the same values. As described above, inimage-data conversion processing in the image data conversion unit 30,for each pixel, the hue H holds the same value and the saturation Sholds the same value in the HSV color space, between the input imagedata D1 and the driving image data D2.

The image display device 3 according to the embodiment obtains thedistribution ratio WRs and the coefficient Ks based on the saturation Sand the parameter WRX and obtains the driving image data D2 with thedistribution ratio WRs and the coefficient Ks which have been obtained.Thus, according to the image display device 3, since the preferableparameter WRX depending on the response characteristics and the like ofthe display unit 40 is set, and the gradation is displayed at a fasterresponse rate, it is possible to improve color reproduction.

In the embodiment, in the distribution ratio-and-coefficient computationunit 32, for each pixel, the first distribution ratio WRs1 is obtainedbased on the saturation S and the parameter WRX such that driving imagedata (white image data Wd) corresponding to the common color subframe isin a range from the minimum value Ddmin of driving image data (blueimage data Bd, green image data Gd, and red image data Rd) correspondingto other subframes to the maximum value Ddmax thereof. The distributionratio WRs of the white subframe is determined based on the obtainedfirst distribution ratio WRs1. Thus, it is possible to suppress thechange of the luminance of the pixel 26 in one frame period and toimprove color reproduction of the image display device. The image dataconversion unit 30 obtains the distribution ratio WRs and the adjustmentcoefficient Ks by the functions which smoothly changes depending on thesaturation S (see FIGS. 16, 24, and the like). Thus, it is possible toprevent distortion of an image when a gradation image is displayed.

In the embodiment, the distribution ratio-and-coefficient computationunit 32 obtains the maximum value allowed to be taken by the coefficientKs, based on the saturation S and the parameter WRX, as the coefficientKs. The distribution ratio-and-coefficient computation unit obtains themaximum value under a condition in which the distribution ratio WRs isused, and the maximum value Dmax of input image data D1 is set toMaximum Value 1 allowed to be taken by the input image data D1, for eachpixel. Thus, it is possible to obtain a large coefficient Ks in anallowable range and to perform amplification and compression on theinput image data D1 in an allowable range.

In the image display device 3 according to the embodiment, the parameterstorage unit 31 stores the parameter WBR for designating the luminanceof the light source 27 provided in the display unit 40 when the commoncolor subframe (white subframe) is displayed, in addition to theparameter WRX. The display unit 40 controls the luminance of the lightsource 27 in accordance with the parameter WBR, when displaying a commoncolor subframe. Thus, according to the embodiment, since the preferableparameter WRX depending on the response characteristics of the displayunit 40 is set, it is possible to improve color reproduction, and toreduce heat generated by the light source 27 by controlling theluminance of the light source 27 of when a common color subframe isdisplayed, with the parameter WBR.

3. Third Embodiment

FIG. 26 is a block diagram illustrating a configuration of an imagedisplay device according to a third embodiment. An image display device5 illustrated in FIG. 26 includes an image data conversion unit 50 and adisplay unit 60. The image data conversion unit 50 is obtained by addinga parameter selection unit 52 to the image data conversion unit 30according to the second embodiment and replacing the parameter storageunit 31 with a parameter storage unit 51. The display unit 60 isobtained by adding a temperature sensor 61 to the display unit 40according to the second embodiment. Differences from the firstembodiment will be described below.

The temperature sensor 61 is provided in the display unit 60 andmeasures the temperature T of the display unit 60. The temperaturesensor 61 is provided, for example, in the vicinity of the liquidcrystal panel 24. The temperature T measured by the temperature sensor61 is input to the parameter selection unit 52.

The parameter storage unit 51 stores a plurality of values for theparameters WRX, RA, RB, WBR, and RC, in accordance with the temperature.The parameter selection unit 52 selects values from the plurality ofvalues stored in the parameter storage unit 51, in accordance with thetemperature T measured by the temperature sensor 61. Then, the parameterselection unit outputs the selected values as the parameters WRX, RA,RB, WBR, and RC. The parameters WRX, RA, RB, WBR, and RC output from theparameter selection unit 52 are input to the distributionratio-and-coefficient computation unit 32. The parameter WBR is alsoinput to the backlight driving circuit 23. The parameters VCBU, GL, andNR pass through the parameter selection unit 52 from the parameterstorage unit 51 and then are input to the distributionratio-and-coefficient computation unit 32.

As described above, in the image display device 5 according to theembodiment, the image data conversion unit 50 includes the parameterstorage unit 51 that stores the parameters WRX, RA, RB, WBR, WRZ0, WRZ1,WRY, GL, RC, and NR used in conversion processing (image-data conversionprocessing). The display unit 60 includes the temperature sensor 61. Theparameter storage unit 51 stores the plurality of values for theparameters WRZ0, WRZ1, WRY, WRX, RA, RB, WBR, and RC in accordance withthe temperature. The image data conversion unit 50 selects valuesdepending on the temperature T measured by the temperature sensor 61,among the plurality of values stored in the parameter storage unit 51.The selected values are used in the conversion processing. Thus,according to the image display device 5, the conversion processing isperformed based on the parameters WRZ0, WRZ1, WRY, WRX, RA, RB, WBR, andRC in accordance with the temperature T of the display unit 60.Accordingly, it is possible to improve color reproduction even in a casewhere the response characteristics of the display unit 60 changedepending on the temperature.

4. Fourth Embodiment

FIG. 27 is a block diagram illustrating a configuration of an imagedisplay device according to a fourth embodiment. An image display device7 illustrated in FIG. 27 includes an image data conversion unit 70 and adisplay unit 60. The image data conversion unit 70 is obtained by addinga frame memory 71 to the image data conversion unit 50 according to thethird embodiment and replacing the statistical value-and-saturationcomputation unit 12 with a statistical value-and-saturation computationunit 72. Differences from the third embodiment will be described below.

Input image data D1 including red image data, green image data, and blueimage data is input to the image display device 7. The frame memory 71stores input image data D1 corresponding to one frame or a plurality offrames.

Similar to the statistical value-and-saturation computation unit 12, thestatistical value-and-saturation computation unit 72 obtains the maximumvalue Dmax, the minimum value Dmin, and the saturation S based on theinput image data D1, for each pixel. At this time, the statisticalvalue-and-saturation computation unit 72 obtains, for each pixel, themaximum value Dmax, the minimum value Dmin, and the saturation S basedon the input image data D1 which has been stored in the frame memory 71and corresponds to a plurality of pixels.

For example, when obtaining the saturation S of a certain pixel, thestatistical value-and-saturation computation unit 72 may obtain thesaturation for a plurality of pixels in the vicinity of this pixel, andobtain an average value, the maximum value, or the minimum value of aplurality of saturations which have been obtained. The statisticalvalue-and-saturation computation unit 72 may perform weighting to thesaturation in the neighboring pixel, in accordance with a distance orthe like from the neighboring pixel and then perform calculation. Thus,since the saturation S is smoothly changed in a spatial direction or thevalue of the adjustment coefficient Ks in accordance with the saturationS is reduced, it is possible to reduce disharmony of an image, whichoccurs by a luminance difference varying depending on the saturation S.The statistical value-and-saturation computation unit 72 may obtain thesaturation S by applying a filter operation to the saturation obtainedfor the previous frame and the saturation obtained for the currentframe. The statistical value-and-saturation computation unit 72 mayperform weighting to the saturation of the previous frame in accordancewith a time difference or the like from the current frame, and thenperform calculation. Thus, since the saturation S is smoothly changed ina time direction or the value of the adjustment coefficient Ks inaccordance with the saturation S is reduced, it is possible to reducedisharmony of an image, which occurs by a luminance difference in thetime direction, which varies depending on the saturation S. Thestatistical value-and-saturation computation unit 72 obtains the maximumvalue Dmax and the minimum value Dmin with the similar methods.

As described above, in the image display device 7 according to theembodiment, the image data conversion unit 70 includes the frame memory71 that stores first image data (input image data D1), and performsconversion processing based on the first image data corresponding to aplurality of pixels stored in the frame memory 71, for each pixel. Thus,according to the image display device 7, it is possible to prevent arapid change of the distribution ratio WRs and the adjustmentcoefficient Ks and to prevent an occurrence of a situation in which thecolor of a pixel 26 rapidly changes in the spatial direction or the timedirection.

5. Modification Example

Regarding the image display device in the embodiments, the followingmodification example can be made. FIG. 28 is a block diagramillustrating a configuration of an image display device according to amodification example of the second embodiment. In an image displaydevice 8 illustrated in FIG. 28, an image data conversion unit 80 isobtained by adding an inverse gamma transformation unit 81, a gammatransformation unit 82, and a response compensation processing unit 83to the image data conversion unit 30 according to the second embodiment.

Input image data D1 to be input to the image display device 8 isgradation data before inverse gamma transformation is performed. Theinverse gamma transformation unit 81 performs inverse gammatransformation on the input image data D1 so as to obtain image data D3after inverse gamma transformation. The parameter storage unit 31, thestatistical value-and-saturation computation unit 12, the distributionratio-and-coefficient computation unit 32, and the driving image-dataoperation unit 33 perform kinds of processing similar to those in thesecond embodiment, on the image data D3 after the inverse gammatransformation, respectively. Thus, image data D4 before gammatransformation is obtained. The gamma transformation unit 82 performsgamma transformation on the image data D4 before the gammatransformation, so as to obtain image data D5. The response compensationprocessing unit 83 performs response compensation processing on theimage data D5 so as to obtain driving image data D2. In the responsecompensation processing unit 83, overdrive processing (may also bereferred to as “overshoot processing”) of compensating for insufficiencyof the response rate of a pixel 26.

In the image display device 8 according to the modification example, theimage data conversion unit 80 obtains driving image data D2 in a mannerthat conversion processing (image-data conversion processing) ofconverting first image data (image data D3 after the inverse gammatransformation) corresponding to a plurality of primary colors intosecond image data (image data D4 before the gamma transformation)corresponding to a plurality of subframes is performed for each pixel,and response compensation processing is performed on image data D5 afterthe conversion processing has been performed. Thus, according to theimage display device 8, it is possible to display a desired image evenin a case where the response rate of the display unit 60 is slow.

The image data conversion unit 80 includes the inverse gammatransformation unit 81, the gamma transformation unit 82, and theresponse compensation processing unit 83. Instead, the image dataconversion unit may include the inverse gamma transformation unit 81 andthe gamma transformation unit 82, but may not include the responsecompensation processing unit 83. Alternatively, the image dataconversion unit may include the response compensation processing unit83, but may not include the inverse gamma transformation unit 81 and thegamma transformation unit 82. At least one of the inverse gammatransformation unit 81 and the gamma transformation unit 82, and theresponse compensation processing unit 83 may be added to the image dataconversion unit 30 according to the second embodiment. The gammatransformation may be performed after the response compensationprocessing. In this case, the response compensation processing isperformed on image data output from the driving image-data operationunit. The gamma transformation is performed on image data after theresponse compensation processing.

In the first to fourth embodiments, the distributionratio-and-coefficient computation unit obtains the coefficient Ks so asto satisfy the expression (1), and thus the expression of RB=1−RA issatisfied (see FIG. 2). Instead, the distribution ratio-and-coefficientcomputation unit may obtain the coefficient Ks such that the minimumvalue DDmin and the maximum value DDmax are in a certain limited rangewhich has been set in a range satisfying 0≤DDmin≤1 and 0≤DDmax≤1. Forexample, the boundary of the limited range illustrated in FIG. 2 is astraight line. However, the boundary of the limited range may be acurved line or a polygonal line having an inflection point. Here, theborder of the limited range is preferably a straight line or a curvedline.

In the first to fourth embodiments, the image display device thatobtains the distribution ratio WRs and the coefficients Ks and Ksv byspecific calculation expressions is described. However, as thecalculation expressions of obtaining the distribution ratio WRs and thecoefficients Ks and Ksv, expressions other than the calculationexpressions described in the embodiments may be provided. For example,as the calculation expression of obtaining the distribution ratio WRs, acalculation expression which has been known from the past may be used.As the calculation expression of obtaining the coefficient Ksv, anycalculation expression satisfying the expression (47) may be used.

Hitherto, the image display devices according to the first to fourthembodiments and the modification example thereof are described. Anycombination of the features of the image display devices according tothe first to fourth embodiments and the modification example thereof aslong as the features do not contradict the properties thereof canconstitute image display devices according to various modificationexamples.

In the first to fourth embodiments, an image is displayed by controllingtransmittance of the liquid crystal panel 24 in which the liquid crystalpanel 24 that causes light from the backlight 25 as the light sourceunit to be transmitted therethrough is used as the display device.However, the present invention is not limited to a field sequentialdisplay device using a transmission type optical modulator as with theliquid crystal panel 24. The present invention can also be applied to afield sequential display device using a reflection type opticalmodulator. For example, the present invention can also be applied to afield sequential projection type display device in which a reflectiontype liquid crystal panel called as a liquid crystal-on-silicon (LCOS)is used as an optical modulator. The present invention can be applied toa field sequential image display device other than a liquid crystaldisplay apparatus, for example, a spontaneous-emission image displaydevice such as an organic electroluminescence (EL) display device, asee-through image display device having a function of seeing through theback of the display panel, or the like.

In the first to fourth embodiments, each frame period is configured withprimary-color subframe periods of the blue color, the green color, andthe red color and the white subframe period (subframe having a whitecolor which is a common color of blue, green, and red) as thecommon-color subframe period. Instead, each frame period may beconfigured with a subframe period of another primary color and thecommon-color subframe period. In this specification, it is assumed that“the common color” means a color including all color components ofprimary colors corresponding to primary-color subframe periods in eachframe period, and the ratio of the color components is not limited. Froma viewpoint that the occurrence of color breakup is suppressed by thecommon color subframe, a common-color subframe period (for example, asubframe period of a yellow color configured with red and green)corresponding to another color configured with two primary colors may beused as the white subframe period as the common-color subframe period.From a similar viewpoint, any color other than black, for example,“yellowish green”, “red”, or “red having the half luminance” can becaused to correspond to the common-color subframe period instead of“white” or “yellow”.

6. Others

This application claims priority right based on Japanese PatentApplication No. 2016-223886 entitled “field sequential image displaydevice and image display method” filed on Nov. 17, 2016, and thecontents of this Japanese application are included in the presentapplication by reference.

REFERENCE SIGNS LIST

-   -   1, 3, 5, 7, 8 IMAGE DISPLAY DEVICE    -   10, 30, 50, 70, 80 IMAGE DATA CONVERSION UNIT    -   20, 40, 60 DISPLAY UNIT    -   11, 31, 51 PARAMETER STORAGE UNIT    -   12, 72 STATISTICAL VALUE-AND-SATURATION COMPUTATION UNIT    -   13, 32 DISTRIBUTION RATIO-AND-COEFFICIENT COMPUTATION UNIT    -   33 DRIVING IMAGE-DATA OPERATION UNIT    -   21 TIMING CONTROL CIRCUIT    -   22 PANEL DRIVING CIRCUIT (LIGHT-MODULATION-UNIT DRIVING CIRCUIT)    -   23 BACKLIGHT DRIVING CIRCUIT (LIGHT-SOURCE-UNIT DRIVING CIRCUIT)    -   24 LIQUID CRYSTAL PANEL (LIGHT MODULATION UNIT)    -   25 BACKLIGHT (LIGHT SOURCE UNIT)    -   26 PIXEL    -   27 LIGHT SOURCE    -   52 PARAMETER SELECTION UNIT    -   61 TEMPERATURE SENSOR    -   71 FRAME MEMORY    -   81 INVERSE GAMMA TRANSFORMATION UNIT    -   82 GAMMA TRANSFORMATION UNIT    -   83 RESPONSE COMPENSATION PROCESSING UNIT    -   Ks ADJUSTMENT COEFFICIENT    -   WRs DISTRIBUTION RATIO OF WHITE SUBFRAME (COMMON-COLOR        DISTRIBUTION RATIO)    -   WRs1 FIRST DISTRIBUTION RATIO (TENTATIVE DISTRIBUTION RATIO)    -   WRsv2 SECOND DISTRIBUTION RATIO

1. A field sequential image display device in which a plurality of subframe periods including a plurality of primary-color subframe periods respectively corresponding to a plurality of primary colors and at least one common-color subframe period is included in each frame period, the device comprising: an image data conversion unit that receives input image data corresponding to the plurality of primary colors and generates driving image data corresponding to the plurality of subframe periods from the input image data by obtaining a pixel data value of each of the plurality of subframe periods for each pixel of an input image represented by the input image data, based on the input image data; and a display unit that displays an image based on the driving image data, wherein the image data conversion unit generates the driving image data by conversion processing in which, for each pixel in the input image, an adjustment coefficient to be multiplied by a value of the pixel and a common-color distribution ratio are determined, and a pixel data value in each of the plurality of subframe periods is obtained from the value of the pixel based on the adjustment coefficient and the common-color distribution ratio, the common-color distribution ratio being defined as a ratio of a display light quantity of a common color component, which is to be emitted in the common-color subframe period to a display light quantity of the common color component, which is to be emitted in one frame period for displaying the pixel; and in the conversion processing, for each pixel in the input image, the common-color distribution ratio is determined in accordance with the saturation and an adjusted brightness of the pixel such that the common-color distribution ratio increases as a hue and the saturation in an HSV space are maintained, and the adjusted brightness decreases, and the pixel is allowed to be displayed in the display unit, the adjusted brightness being a brightness after the value of the pixel is multiplied by the adjustment coefficient.
 2. The image display device according to claim 1, wherein, for each pixel in the input image, the image data conversion unit determines a tentative distribution ratio corresponding to the ratio in accordance with the saturation of the pixel such that the pixel data value in the common-color subframe period is greater than a minimum value of pixel data values in the plurality of primary-color subframe periods and smaller than a maximum value thereof, and determines the common-color distribution ratio based on the tentative distribution ratio in accordance with the adjusted brightness of the pixel.
 3. The image display device according to claim 1, wherein the image data conversion unit includes a parameter storage unit that stores a parameter used in the conversion processing, and the parameter storage unit stores a parameter in accordance with response characteristics in image display in the display unit.
 4. The image display device according to claim 3, wherein the image data conversion unit further stores a parameter for designating a range of a maximum value in accordance with a minimum value of pixel data values of each pixel in the input image in the plurality of subframe periods.
 5. The image display device according to claim 3, wherein the display unit includes a light source unit that emits light having a corresponding color in each subframe period, a light modulation unit that cause the light from the light source unit to be transmitted therethrough or be reflected thereby, a light-source-unit driving circuit that drives the light source unit to irradiate the light modulation unit with the light having the corresponding color in each subframe period, and a light-modulation-unit driving circuit that controls transmittance or reflectance in the light modulation unit such that an image of the corresponding color in each subframe period is displayed, the parameter storage unit further stores a light emission control parameter, and the light-source-unit driving circuit controls light emission luminance of a common color component in the light source unit based on the light emission control parameter.
 6. The image display device according to claim 1, wherein the image data conversion unit obtains the common-color distribution ratio in accordance with a function having a value which smoothly changes depending on the saturation.
 7. The image display device according to claim 1, wherein the image data conversion unit generates the driving image data by conversion processing in which, for each pixel in the input image, the adjustment coefficient is determined based on pixel data values in the plurality of subframe periods in accordance with the saturation of the pixel in a range in which the pixel is allowed to be displayed in the display unit, and the pixel data value in each of the plurality of subframe periods is obtained from the value of the pixel based on the adjustment coefficient and the common-color distribution ratio.
 8. The image display device according to claim 7, wherein the image data conversion unit obtains the common-color distribution ratio and the adjustment coefficient in accordance with functions having a value which smoothly changes depending on the saturation.
 9. The image display device according to claim 7, wherein the image data conversion unit determines the adjustment coefficient and the common-color distribution ratio such that a maximum value is linearly limited with respect to a minimum value among pixel data values in the plurality of subframe periods, for each pixel in the input image.
 10. The image display device according to claim 7, wherein the image data conversion unit assumes a function of the saturation, which indicates a tentative coefficient for obtaining the adjustment coefficient and a function of the saturation, which indicates a correction coefficient to be multiplied by the tentative coefficient, and obtains a multiplication result of the tentative coefficient and the correction coefficient based on the saturation of the pixel for each pixel in the input image, as the adjustment coefficient, and the correction coefficient is set such that a rate of the adjustment coefficient changing with respect to the saturation when the saturation is equal to or smaller than a predetermined value is equal to or smaller than a predetermined value.
 11. The image display device according to claim 1, wherein the image data conversion unit includes a parameter storage unit that stores a parameter used in the conversion processing, the display unit includes a temperature sensor, the parameter storage unit stores a plurality of values for the parameter, in accordance with a temperature, and the image data conversion unit selects the value in accordance with the temperature measured by the temperature sensor among the plurality of values stored in the parameter storage unit and uses the selected value in the conversion processing.
 12. The image display device according to claim 1, wherein the image data conversion unit includes a frame memory that stores the input image data, and generates the driving image data corresponding to a pixel, based on the input image data which has been stored in the frame memory and corresponds to a plurality of pixels, for each pixel in the input image.
 13. The image display device according to claim 1, wherein the image data conversion unit performs the conversion processing on normalized luminance data.
 14. The image display device according to claim 1, wherein the image data conversion unit obtains the driving image data by performing response compensation processing on image data obtained after the conversion processing.
 15. The image display device according to claim 1, wherein the plurality of primary colors includes blue, green, and red, and a common color is white.
 16. A field sequential image display method in which a plurality of subframe periods including a plurality of primary-color subframe periods respectively corresponding to a plurality of primary colors and at least one common-color subframe period is included in each frame period, the method comprising: an image-data conversion step of receiving input image data corresponding to the plurality of primary colors and generating driving image data corresponding to the plurality of subframe periods from the input image data by obtaining a pixel data value of each of the plurality of subframe periods for each pixel of an input image represented by the input image data, based on the input image data; and a display step of displaying an image based on the driving image data, wherein the image-data conversion step includes a coefficient-and-distribution ratio determination step of determining an adjustment coefficient to be multiplied by a value of the pixel and determining a common-color distribution ratio defined as a ratio of a display light quantity of a common color component, which is to be emitted in the common-color subframe period to a display light quantity of the common color component, which is to be emitted in one frame period for displaying the pixel, for each pixel in the input image; and a driving image-data operation step of generating the driving image data by conversion processing of obtaining the pixel data value in each of the plurality of subframe periods from the value of the pixel based on the adjustment coefficient and the common-color distribution ratio, for each pixel in the input image; and in the coefficient-and-distribution ratio determination step, for each pixel in the input image, the common-color distribution ratio is determined in accordance with the saturation and an adjusted brightness of the pixel such that the common-color distribution ratio increases as a hue and the saturation in an HSV space are maintained, and the adjusted brightness decreases, and the pixel is allowed to be displayed in the display unit, the adjusted brightness being a brightness after the value of the pixel is multiplied by the adjustment coefficient. 